diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3e024a3043..e74b950467 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,10 @@
 - Fix: 一部のブラウザでバナー画像が上下中央に表示されない問題を修正
 - Fix: ナビゲーションバーの設定で削除した項目をその場で再追加できない問題を修正
 - Fix: ロールポリシーによりダイレクトメッセージが無効化されている際のデッキのダイレクトメッセージカラムの挙動を改善
+- Fix: ウォーターマークの各種挙動修正
+  - ウォーターマークを回転させると歪む問題を修正
+  - ウォーターマークを敷き詰めると上下左右反転した画像/文字が表示される問題を修正
+  - ウォーターマークを回転させた際に画面からはみ出た部分を考慮できるように
 
 ### Server
 -
diff --git a/locales/index.d.ts b/locales/index.d.ts
index d79db121db..96d6c890a8 100644
--- a/locales/index.d.ts
+++ b/locales/index.d.ts
@@ -12345,6 +12345,10 @@ export interface Locale extends ILocale {
          * 敷き詰める
          */
         "repeat": string;
+        /**
+         * 回転時はみ出ないように調整する
+         */
+        "preserveBoundingRect": string;
         /**
          * 不透明度
          */
diff --git a/locales/ja-JP.yml b/locales/ja-JP.yml
index 5d1c37740c..8e935b5d9e 100644
--- a/locales/ja-JP.yml
+++ b/locales/ja-JP.yml
@@ -3305,6 +3305,7 @@ _watermarkEditor:
   title: "ウォーターマークの編集"
   cover: "全体に被せる"
   repeat: "敷き詰める"
+  preserveBoundingRect: "回転時はみ出ないように調整する"
   opacity: "不透明度"
   scale: "サイズ"
   text: "テキスト"
diff --git a/packages/frontend/src/components/MkWatermarkEditorDialog.Layer.vue b/packages/frontend/src/components/MkWatermarkEditorDialog.Layer.vue
index 288293db3f..b34181e5cc 100644
--- a/packages/frontend/src/components/MkWatermarkEditorDialog.Layer.vue
+++ b/packages/frontend/src/components/MkWatermarkEditorDialog.Layer.vue
@@ -65,6 +65,10 @@ SPDX-License-Identifier: AGPL-3.0-only
 		<MkSwitch v-model="layer.repeat">
 			<template #label>{{ i18n.ts._watermarkEditor.repeat }}</template>
 		</MkSwitch>
+
+		<MkSwitch v-model="layerPreserveBoundingRect">
+			<template #label>{{ i18n.ts._watermarkEditor.preserveBoundingRect }}</template>
+		</MkSwitch>
 	</template>
 
 	<template v-else-if="layer.type === 'image'">
@@ -129,6 +133,10 @@ SPDX-License-Identifier: AGPL-3.0-only
 		<MkSwitch v-model="layer.cover">
 			<template #label>{{ i18n.ts._watermarkEditor.cover }}</template>
 		</MkSwitch>
+
+		<MkSwitch v-model="layerPreserveBoundingRect">
+			<template #label>{{ i18n.ts._watermarkEditor.preserveBoundingRect }}</template>
+		</MkSwitch>
 	</template>
 
 	<template v-else-if="layer.type === 'qr'">
@@ -335,7 +343,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 </template>
 
 <script setup lang="ts">
-import { ref, onMounted } from 'vue';
+import { ref, onMounted, computed } from 'vue';
 import * as Misskey from 'misskey-js';
 import type { WatermarkPreset } from '@/utility/watermark.js';
 import { i18n } from '@/i18n.js';
@@ -351,6 +359,20 @@ import { misskeyApi } from '@/utility/misskey-api.js';
 
 const layer = defineModel<WatermarkPreset['layers'][number]>('layer', { required: true });
 
+const layerPreserveBoundingRect = computed({
+	get: () => {
+		if (layer.value.type === 'text' || layer.value.type === 'image') {
+			return !layer.value.noBoundingBoxExpansion;
+		}
+		return false;
+	},
+	set: (v: boolean) => {
+		if (layer.value.type === 'text' || layer.value.type === 'image') {
+			layer.value.noBoundingBoxExpansion = !v;
+		}
+	},
+});
+
 const driveFile = ref<Misskey.entities.DriveFile | null>(null);
 const driveFileError = ref(false);
 onMounted(async () => {
diff --git a/packages/frontend/src/components/MkWatermarkEditorDialog.vue b/packages/frontend/src/components/MkWatermarkEditorDialog.vue
index 0d0488d9bc..3b3f20d8d1 100644
--- a/packages/frontend/src/components/MkWatermarkEditorDialog.vue
+++ b/packages/frontend/src/components/MkWatermarkEditorDialog.vue
@@ -90,6 +90,7 @@ function createTextLayer(): WatermarkPreset['layers'][number] {
 		angle: 0,
 		opacity: 0.75,
 		repeat: false,
+		noBoundingBoxExpansion: false,
 	};
 }
 
@@ -104,6 +105,7 @@ function createImageLayer(): WatermarkPreset['layers'][number] {
 		angle: 0,
 		opacity: 0.75,
 		repeat: false,
+		noBoundingBoxExpansion: false,
 		cover: false,
 	};
 }
diff --git a/packages/frontend/src/utility/image-effector/fxs/watermarkPlacement.ts b/packages/frontend/src/utility/image-effector/fxs/watermarkPlacement.ts
index f79acb44b0..944e790792 100644
--- a/packages/frontend/src/utility/image-effector/fxs/watermarkPlacement.ts
+++ b/packages/frontend/src/utility/image-effector/fxs/watermarkPlacement.ts
@@ -9,80 +9,143 @@ const shader = `#version 300 es
 precision mediump float;
 
 const float PI = 3.141592653589793;
-const float TWO_PI = 6.283185307179586;
-const float HALF_PI = 1.5707963267948966;
 
-in vec2 in_uv;
-uniform sampler2D in_texture;
-uniform vec2 in_resolution;
-uniform sampler2D u_texture_watermark;
-uniform vec2 u_resolution_watermark;
-uniform float u_scale;
-uniform float u_angle;
-uniform float u_opacity;
-uniform bool u_repeat;
-uniform int u_alignX; // 0: left, 1: center, 2: right
-uniform int u_alignY; // 0: top, 1: center, 2: bottom
-uniform float u_alignMargin;
-uniform int u_fitMode; // 0: contain, 1: cover
+in vec2 in_uv;                       // 0..1
+uniform sampler2D in_texture;        // 背景
+uniform vec2 in_resolution;          // 出力解像度(px)
+
+uniform sampler2D u_watermark;       // ウォーターマーク
+uniform vec2 u_wmResolution;         // ウォーターマーク元解像度(px)
+
+uniform float u_opacity;             // 0..1
+uniform float u_scale;               // watermarkのスケール
+uniform float u_angle;               // -1..1 (PI倍)
+uniform bool u_cover;                // cover基準 or fit基準
+uniform bool u_repeat;               // タイル敷き詰め
+uniform int u_alignX;                // 0:left 1:center 2:right
+uniform int u_alignY;                // 0:top 1:center 2:bottom
+uniform float u_margin;              // 余白(比率)
+uniform float u_repeatMargin;        // 敷き詰め時の余白(比率)
+uniform bool u_noBBoxExpansion;      // 回転時のBounding Box拡張を抑止
+uniform bool u_wmEnabled;            // watermark有効
+
 out vec4 out_color;
 
+mat2 rot(float a) {
+	float c = cos(a), s = sin(a);
+	return mat2(c, -s, s, c);
+}
+
+// cover/fitとscaleから、最終的なサイズ(px)を計算
+vec2 computeWmSize(vec2 outSize, vec2 wmSize, bool cover, float scale) {
+	float wmAspect = wmSize.x / wmSize.y;
+	float outAspect = outSize.x / outSize.y;
+	vec2 size;
+	if (cover) {
+		if (wmAspect >= outAspect) {
+			size.y = outSize.y * scale;
+			size.x = size.y * wmAspect;
+		} else {
+			size.x = outSize.x * scale;
+			size.y = size.x / wmAspect;
+		}
+	} else {
+		if (wmAspect >= outAspect) {
+			size.x = outSize.x * scale;
+			size.y = size.x / wmAspect;
+		} else {
+			size.y = outSize.y * scale;
+			size.x = size.y * wmAspect;
+		}
+	}
+	return size;
+}
+
 void main() {
-	vec4 in_color = texture(in_texture, in_uv);
-	float in_x_ratio = max(in_resolution.x / in_resolution.y, 1.0);
-	float in_y_ratio = max(in_resolution.y / in_resolution.x, 1.0);
+	vec2 outSize = in_resolution;
+	vec2 p = in_uv * outSize; // 出力のピクセル座標
+	vec4 base = texture(in_texture, in_uv);
 
-	bool contain = u_fitMode == 0;
+	if (!u_wmEnabled) {
+		out_color = base;
+		return;
+	}
 
-	float x_ratio = u_resolution_watermark.x / in_resolution.x;
-	float y_ratio = u_resolution_watermark.y / in_resolution.y;
+	float theta = u_angle * PI; // ラジアン
+	vec2 wmSize = computeWmSize(outSize, u_wmResolution, u_cover, u_scale);
+	vec2 margin = u_repeat ? wmSize * u_repeatMargin : outSize * u_margin;
 
-	float aspect_ratio = contain ?
-		(min(x_ratio, y_ratio) / max(x_ratio, y_ratio)) :
-		(max(x_ratio, y_ratio) / min(x_ratio, y_ratio));
+	// アライメントに基づく回転中心を計算
+	float rotateX = 0.0;
+	float rotateY = 0.0;
+	if (abs(theta) > 1e-6 && !u_noBBoxExpansion) {
+		rotateX = abs(abs(wmSize.x * cos(theta)) + abs(wmSize.y * sin(theta)) - wmSize.x) * 0.5;
+		rotateY = abs(abs(wmSize.x * sin(theta)) + abs(wmSize.y * cos(theta)) - wmSize.y) * 0.5;
+	}
 
-	float x_scale = contain ?
-		(x_ratio > y_ratio ? 1.0 * u_scale : aspect_ratio * u_scale) :
-		(x_ratio > y_ratio ? aspect_ratio * u_scale : 1.0 * u_scale);
+	float x;
+	if (u_alignX == 1) {
+		x = (outSize.x - wmSize.x) * 0.5;
+	} else if (u_alignX == 0) {
+		x = rotateX + margin.x;
+	} else {
+		x = outSize.x - wmSize.x - margin.x - rotateX;
+	}
 
-	float y_scale = contain ?
-		(y_ratio > x_ratio ? 1.0 * u_scale : aspect_ratio * u_scale) :
-		(y_ratio > x_ratio ? aspect_ratio * u_scale : 1.0 * u_scale);
+	float y;
+	if (u_alignY == 1) {
+		y = (outSize.y - wmSize.y) * 0.5;
+	} else if (u_alignY == 0) {
+		y = rotateY + margin.y;
+	} else {
+		y = outSize.y - wmSize.y - margin.y - rotateY;
+	}
 
-	float x_offset = u_alignX == 0 ? x_scale / 2.0 : u_alignX == 2 ? 1.0 - (x_scale / 2.0) : 0.5;
-	float y_offset = u_alignY == 0 ? y_scale / 2.0 : u_alignY == 2 ? 1.0 - (y_scale / 2.0) : 0.5;
+	vec2 rectMin = vec2(x, y);
+	vec2 rectMax = rectMin + wmSize;
+	vec2 rectCenter = (rectMin + rectMax) * 0.5;
 
-	x_offset += (u_alignX == 0 ? 1.0 : u_alignX == 2 ? -1.0 : 0.0) * u_alignMargin;
-	y_offset += (u_alignY == 0 ? 1.0 : u_alignY == 2 ? -1.0 : 0.0) * u_alignMargin;
+	vec4 wmCol = vec4(0.0);
 
-	float angle = -(u_angle * PI);
-	vec2 center = vec2(x_offset, y_offset);
-	//vec2 centeredUv = (in_uv - center) * vec2(in_x_ratio, in_y_ratio);
-	vec2 centeredUv = (in_uv - center);
-	vec2 rotatedUV = vec2(
-		centeredUv.x * cos(angle) - centeredUv.y * sin(angle),
-		centeredUv.x * sin(angle) + centeredUv.y * cos(angle)
-	) + center;
+	if (u_repeat) {
+		// アライメントに基づく中心で回転
+		vec2 q = rectCenter + rot(theta) * (p - rectCenter);
 
-	// trim
-	if (!u_repeat) {
-		bool isInside = rotatedUV.x > x_offset - (x_scale / 2.0) && rotatedUV.x < x_offset + (x_scale / 2.0) &&
-										rotatedUV.y > y_offset - (y_scale / 2.0) && rotatedUV.y < y_offset + (y_scale / 2.0);
-		if (!isInside) {
-			out_color = in_color;
-			return;
+		// タイルグリッドの原点をrectMin（アライメント位置）に設定
+		vec2 gridOrigin = rectMin - margin;
+		vec2 qFromOrigin = q - gridOrigin;
+
+		// タイルサイズ(ウォーターマーク + マージン)で正規化
+		vec2 tile = wmSize + margin * 2.0;
+		vec2 tileUv = qFromOrigin / tile;
+
+		// タイル内のローカル座標(0..1)を取得
+		vec2 localUv = fract(tileUv);
+
+		// ローカル座標をピクセル単位に変換
+		vec2 localPos = localUv * tile;
+
+		// マージン領域内かチェック
+		bool inMargin = any(lessThan(localPos, margin)) || any(greaterThanEqual(localPos, margin + wmSize));
+
+		if (!inMargin) {
+			// ウォーターマーク領域内: UV座標を計算
+			vec2 uvWm = (localPos - margin) / wmSize;
+			wmCol = texture(u_watermark, uvWm);
+		}
+		// マージン領域の場合は透明(wmCol = vec4(0.0))のまま
+	} else {
+		// アライメントと回転に従い一枚だけ描画
+		vec2 q = rectCenter + rot(theta) * (p - rectCenter);
+		bool inside = all(greaterThanEqual(q, rectMin)) && all(lessThan(q, rectMax));
+		if (inside) {
+			vec2 uvWm = (q - rectMin) / wmSize;
+			wmCol = texture(u_watermark, uvWm);
 		}
 	}
 
-	vec4 watermark_color = texture(u_texture_watermark, vec2(
-		(rotatedUV.x - (x_offset - (x_scale / 2.0))) / x_scale,
-		(rotatedUV.y - (y_offset - (y_scale / 2.0))) / y_scale
-	));
-
-	out_color.r = mix(in_color.r, watermark_color.r, u_opacity * watermark_color.a);
-	out_color.g = mix(in_color.g, watermark_color.g, u_opacity * watermark_color.a);
-	out_color.b = mix(in_color.b, watermark_color.b, u_opacity * watermark_color.a);
-	out_color.a = in_color.a * (1.0 - u_opacity * watermark_color.a) + watermark_color.a * u_opacity;
+	float a = clamp(wmCol.a * u_opacity, 0.0, 1.0);
+	out_color = mix(base, vec4(wmCol.rgb, 1.0), a);
 }
 `;
 
@@ -90,7 +153,7 @@ export const FX_watermarkPlacement = defineImageEffectorFx({
 	id: 'watermarkPlacement',
 	name: '(internal)',
 	shader,
-	uniforms: ['texture_watermark', 'resolution_watermark', 'scale', 'angle', 'opacity', 'repeat', 'alignX', 'alignY', 'alignMargin', 'fitMode'] as const,
+	uniforms: ['opacity', 'scale', 'angle', 'cover', 'repeat', 'alignX', 'alignY', 'margin', 'repeatMargin', 'noBBoxExpansion', 'wmResolution', 'wmEnabled', 'watermark'] as const,
 	params: {
 		cover: {
 			type: 'boolean',
@@ -125,29 +188,50 @@ export const FX_watermarkPlacement = defineImageEffectorFx({
 			max: 1.0,
 			step: 0.01,
 		},
+		noBoundingBoxExpansion: {
+			type: 'boolean',
+			default: false,
+		},
 		watermark: {
 			type: 'texture',
 			default: null,
 		},
 	},
 	main: ({ gl, u, params, textures }) => {
-		if (textures.watermark == null) {
-			return;
-		}
-
-		gl.activeTexture(gl.TEXTURE1);
-		gl.bindTexture(gl.TEXTURE_2D, textures.watermark.texture);
-		gl.uniform1i(u.texture_watermark, 1);
-
-		gl.uniform2fv(u.resolution_watermark, [textures.watermark.width, textures.watermark.height]);
-		gl.uniform1f(u.scale, params.scale);
-
-		gl.uniform1f(u.opacity, params.opacity);
-		gl.uniform1f(u.angle, params.angle);
+		// 基本パラメータ
+		gl.uniform1f(u.opacity, params.opacity ?? 1.0);
+		gl.uniform1f(u.scale, params.scale ?? 0.3);
+		gl.uniform1f(u.angle, params.angle ?? 0.0);
+		gl.uniform1i(u.cover, params.cover ? 1 : 0);
 		gl.uniform1i(u.repeat, params.repeat ? 1 : 0);
-		gl.uniform1i(u.alignX, params.align.x === 'left' ? 0 : params.align.x === 'right' ? 2 : 1);
-		gl.uniform1i(u.alignY, params.align.y === 'top' ? 0 : params.align.y === 'bottom' ? 2 : 1);
-		gl.uniform1f(u.alignMargin, params.align.margin ?? 0);
-		gl.uniform1i(u.fitMode, params.cover ? 1 : 0);
+		const ax = params.align?.x === 'left' ? 0 : params.align?.x === 'center' ? 1 : 2;
+		const ay = params.align?.y === 'top' ? 0 : params.align?.y === 'center' ? 1 : 2;
+		gl.uniform1i(u.alignX, ax);
+		gl.uniform1i(u.alignY, ay);
+		gl.uniform1f(u.margin, (params.align?.margin ?? 0));
+		gl.uniform1f(u.repeatMargin, (params.align?.margin ?? 0));
+		gl.uniform1i(u.noBBoxExpansion, params.noBoundingBoxExpansion ? 1 : 0);
+
+		// ウォーターマークテクスチャ
+		const wm = textures.watermark;
+		if (wm) {
+			gl.activeTexture(gl.TEXTURE1);
+			gl.bindTexture(gl.TEXTURE_2D, wm.texture);
+
+			// リピートモードに応じてWRAP属性を設定
+			if (params.repeat) {
+				gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.REPEAT);
+				gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.REPEAT);
+			} else {
+				gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
+				gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
+			}
+
+			gl.uniform1i(u.watermark, 1);
+			gl.uniform2f(u.wmResolution, wm.width, wm.height);
+			gl.uniform1i(u.wmEnabled, 1);
+		} else {
+			gl.uniform1i(u.wmEnabled, 0);
+		}
 	},
 });
diff --git a/packages/frontend/src/utility/watermark.ts b/packages/frontend/src/utility/watermark.ts
index b3525f158f..1b46721a2b 100644
--- a/packages/frontend/src/utility/watermark.ts
+++ b/packages/frontend/src/utility/watermark.ts
@@ -27,6 +27,7 @@ export type WatermarkPreset = {
 		type: 'text';
 		text: string;
 		repeat: boolean;
+		noBoundingBoxExpansion: boolean;
 		scale: number;
 		angle: number;
 		align: Align;
@@ -38,6 +39,7 @@ export type WatermarkPreset = {
 		imageId: string | null;
 		cover: boolean;
 		repeat: boolean;
+		noBoundingBoxExpansion: boolean;
 		scale: number;
 		angle: number;
 		align: Align;
@@ -106,6 +108,7 @@ export class WatermarkRenderer {
 					id: layer.id,
 					params: {
 						repeat: layer.repeat,
+						noBoundingBoxExpansion: layer.noBoundingBoxExpansion,
 						scale: layer.scale,
 						align: layer.align,
 						angle: layer.angle,
@@ -123,6 +126,7 @@ export class WatermarkRenderer {
 					id: layer.id,
 					params: {
 						repeat: layer.repeat,
+						noBoundingBoxExpansion: layer.noBoundingBoxExpansion,
 						scale: layer.scale,
 						align: layer.align,
 						angle: layer.angle,