diff --git a/src/core/terrain-brush.ts b/src/core/terrain-brush.ts
new file mode 100644
index 00000000..a4271806
--- /dev/null
+++ b/src/core/terrain-brush.ts
@@ -0,0 +1,91 @@
+import type { Terrain } from "../document/terrains";
+
+export type TerrainBrushTool = "raise" | "lower" | "smooth" | "flatten";
+
+export interface TerrainBrushSettings {
+  radius: number;
+  strength: number;
+  falloff: number;
+}
+
+export interface ArmedTerrainBrushState extends TerrainBrushSettings {
+  terrainId: string;
+  tool: TerrainBrushTool;
+}
+
+export interface TerrainBrushStrokeCommit {
+  terrain: Terrain;
+  commandLabel: string;
+  tool: TerrainBrushTool;
+}
+
+export const DEFAULT_TERRAIN_BRUSH_RADIUS = 2;
+export const DEFAULT_TERRAIN_BRUSH_STRENGTH = 0.35;
+export const DEFAULT_TERRAIN_BRUSH_FALLOFF = 0.6;
+export const MIN_TERRAIN_BRUSH_RADIUS = 0.25;
+export const MAX_TERRAIN_BRUSH_RADIUS = 12;
+export const MIN_TERRAIN_BRUSH_STRENGTH = 0.05;
+export const MAX_TERRAIN_BRUSH_STRENGTH = 1;
+export const MIN_TERRAIN_BRUSH_FALLOFF = 0;
+export const MAX_TERRAIN_BRUSH_FALLOFF = 1;
+
+function clamp(value: number, min: number, max: number): number {
+  return Math.min(max, Math.max(min, value));
+}
+
+export function clampTerrainBrushRadius(radius: number): number {
+  if (!Number.isFinite(radius)) {
+    return DEFAULT_TERRAIN_BRUSH_RADIUS;
+  }
+
+  return clamp(radius, MIN_TERRAIN_BRUSH_RADIUS, MAX_TERRAIN_BRUSH_RADIUS);
+}
+
+export function clampTerrainBrushStrength(strength: number): number {
+  if (!Number.isFinite(strength)) {
+    return DEFAULT_TERRAIN_BRUSH_STRENGTH;
+  }
+
+  return clamp(
+    strength,
+    MIN_TERRAIN_BRUSH_STRENGTH,
+    MAX_TERRAIN_BRUSH_STRENGTH
+  );
+}
+
+export function clampTerrainBrushFalloff(falloff: number): number {
+  if (!Number.isFinite(falloff)) {
+    return DEFAULT_TERRAIN_BRUSH_FALLOFF;
+  }
+
+  return clamp(
+    falloff,
+    MIN_TERRAIN_BRUSH_FALLOFF,
+    MAX_TERRAIN_BRUSH_FALLOFF
+  );
+}
+
+export function createDefaultTerrainBrushSettings(): TerrainBrushSettings {
+  return {
+    radius: DEFAULT_TERRAIN_BRUSH_RADIUS,
+    strength: DEFAULT_TERRAIN_BRUSH_STRENGTH,
+    falloff: DEFAULT_TERRAIN_BRUSH_FALLOFF
+  };
+}
+
+export function getTerrainBrushToolLabel(tool: TerrainBrushTool): string {
+  switch (tool) {
+    case "raise":
+      return "Raise";
+    case "lower":
+      return "Lower";
+    case "smooth":
+      return "Smooth";
+    case "flatten":
+      return "Flatten";
+  }
+}
+
+export function getTerrainBrushCommandLabel(tool: TerrainBrushTool): string {
+  return `${getTerrainBrushToolLabel(tool)} terrain`;
+}
diff --git a/src/geometry/terrain-brush.ts b/src/geometry/terrain-brush.ts
new file mode 100644
index 00000000..73d1f5ea
--- /dev/null
+++ b/src/geometry/terrain-brush.ts
@@ -0,0 +1,253 @@
+import type { Vec3 } from "../core/vector";
+import type {
+  TerrainBrushSettings,
+  TerrainBrushTool
+} from "../core/terrain-brush";
+import {
+  createTerrain,
+  getTerrainHeightAtSample,
+  getTerrainSampleIndex,
+  type Terrain
+} from "../document/terrains";
+
+export interface TerrainBrushPoint {
+  x: number;
+  z: number;
+}
+
+function clamp(value: number, min: number, max: number): number {
+  return Math.min(max, Math.max(min, value));
+}
+
+function lerp(start: number, end: number, t: number): number {
+  return start + (end - start) * t;
+}
+
+function clamp01(value: number): number {
+  return clamp(value, 0, 1);
+}
+
+export function getTerrainBrushWeight(
+  distance: number,
+  radius: number,
+  falloff: number
+): number {
+  if (!Number.isFinite(radius) || radius <= 0) {
+    return 0;
+  }
+
+  if (distance >= radius) {
+    return 0;
+  }
+
+  const normalizedDistance = clamp01(distance / radius);
+  const baseWeight = 1 - normalizedDistance;
+  const exponent = 1 + clamp01(falloff) * 3;
+  return Math.pow(baseWeight, exponent);
+}
+
+export function sampleTerrainHeightAtWorldPosition(
+  terrain: Terrain,
+  worldX: number,
+  worldZ: number,
+  clampToBounds = false
+): number | null {
+  const sampleSpaceX = (worldX - terrain.position.x) / terrain.cellSize;
+  const sampleSpaceZ = (worldZ - terrain.position.z) / terrain.cellSize;
+  const maxSampleX = terrain.sampleCountX - 1;
+  const maxSampleZ = terrain.sampleCountZ - 1;
+
+  if (!clampToBounds) {
+    if (
+      sampleSpaceX < 0 ||
+      sampleSpaceX > maxSampleX ||
+      sampleSpaceZ < 0 ||
+      sampleSpaceZ > maxSampleZ
+    ) {
+      return null;
+    }
+  }
+
+  const clampedSampleX = clamp(sampleSpaceX, 0, maxSampleX);
+  const clampedSampleZ = clamp(sampleSpaceZ, 0, maxSampleZ);
+  const x0 = Math.floor(clampedSampleX);
+  const z0 = Math.floor(clampedSampleZ);
+  const x1 = Math.min(maxSampleX, x0 + 1);
+  const z1 = Math.min(maxSampleZ, z0 + 1);
+  const tx = clampedSampleX - x0;
+  const tz = clampedSampleZ - z0;
+  const top = lerp(
+    getTerrainHeightAtSample(terrain, x0, z0),
+    getTerrainHeightAtSample(terrain, x1, z0),
+    tx
+  );
+  const bottom = lerp(
+    getTerrainHeightAtSample(terrain, x0, z1),
+    getTerrainHeightAtSample(terrain, x1, z1),
+    tx
+  );
+
+  return lerp(top, bottom, tz);
+}
+
+export function createTerrainBrushPreviewPoints(
+  terrain: Terrain,
+  center: TerrainBrushPoint,
+  radius: number,
+  segmentCount = 36,
+  heightOffset = 0.06
+): Vec3[] {
+  const points: Vec3[] = [];
+  const minX = terrain.position.x;
+  const minZ = terrain.position.z;
+  const maxX =
+    terrain.position.x + (terrain.sampleCountX - 1) * terrain.cellSize;
+  const maxZ =
+    terrain.position.z + (terrain.sampleCountZ - 1) * terrain.cellSize;
+
+  for (let segmentIndex = 0; segmentIndex <= segmentCount; segmentIndex += 1) {
+    const angle = (segmentIndex / segmentCount) * Math.PI * 2;
+    const unclampedX = center.x + Math.cos(angle) * radius;
+    const unclampedZ = center.z + Math.sin(angle) * radius;
+    const worldX = clamp(unclampedX, minX, maxX);
+    const worldZ = clamp(unclampedZ, minZ, maxZ);
+    const height =
+      sampleTerrainHeightAtWorldPosition(terrain, worldX, worldZ, true) ?? 0;
+
+    points.push({
+      x: worldX,
+      y: terrain.position.y + height + heightOffset,
+      z: worldZ
+    });
+  }
+
+  return points;
+}
+
+function getTerrainSmoothTargetHeight(
+  terrain: Terrain,
+  sourceHeights: readonly number[],
+  sampleX: number,
+  sampleZ: number
+): number {
+  let total = 0;
+  let count = 0;
+
+  for (
+    let neighborZ = Math.max(0, sampleZ - 1);
+    neighborZ <= Math.min(terrain.sampleCountZ - 1, sampleZ + 1);
+    neighborZ += 1
+  ) {
+    for (
+      let neighborX = Math.max(0, sampleX - 1);
+      neighborX <= Math.min(terrain.sampleCountX - 1, sampleX + 1);
+      neighborX += 1
+    ) {
+      total +=
+        sourceHeights[getTerrainSampleIndex(terrain, neighborX, neighborZ)] ?? 0;
+      count += 1;
+    }
+  }
+
+  return count === 0
+    ? sourceHeights[getTerrainSampleIndex(terrain, sampleX, sampleZ)] ?? 0
+    : total / count;
+}
+
+export function applyTerrainBrushStamp(options: {
+  terrain: Terrain;
+  center: TerrainBrushPoint;
+  settings: TerrainBrushSettings;
+  tool: TerrainBrushTool;
+  referenceHeight?: number | null;
+}): Terrain {
+  const { terrain, center, settings, tool, referenceHeight = null } = options;
+  const { radius, strength, falloff } = settings;
+  const minSampleX = Math.max(
+    0,
+    Math.floor((center.x - terrain.position.x - radius) / terrain.cellSize)
+  );
+  const maxSampleX = Math.min(
+    terrain.sampleCountX - 1,
+    Math.ceil((center.x - terrain.position.x + radius) / terrain.cellSize)
+  );
+  const minSampleZ = Math.max(
+    0,
+    Math.floor((center.z - terrain.position.z - radius) / terrain.cellSize)
+  );
+  const maxSampleZ = Math.min(
+    terrain.sampleCountZ - 1,
+    Math.ceil((center.z - terrain.position.z + radius) / terrain.cellSize)
+  );
+  const sourceHeights = terrain.heights;
+  const nextHeights = [...sourceHeights];
+  const smoothingStrength = clamp01(strength);
+  let changed = false;
+
+  for (let sampleZ = minSampleZ; sampleZ <= maxSampleZ; sampleZ += 1) {
+    for (let sampleX = minSampleX; sampleX <= maxSampleX; sampleX += 1) {
+      const worldX = terrain.position.x + sampleX * terrain.cellSize;
+      const worldZ = terrain.position.z + sampleZ * terrain.cellSize;
+      const distance = Math.hypot(worldX - center.x, worldZ - center.z);
+      const weight = getTerrainBrushWeight(distance, radius, falloff);
+
+      if (weight <= 0) {
+        continue;
+      }
+
+      const sampleIndex = getTerrainSampleIndex(terrain, sampleX, sampleZ);
+      const currentHeight = sourceHeights[sampleIndex] ?? 0;
+      let nextHeight = currentHeight;
+
+      switch (tool) {
+        case "raise":
+          nextHeight = currentHeight + strength * weight;
+          break;
+        case "lower":
+          nextHeight = currentHeight - strength * weight;
+          break;
+        case "smooth": {
+          const smoothTargetHeight = getTerrainSmoothTargetHeight(
+            terrain,
+            sourceHeights,
+            sampleX,
+            sampleZ
+          );
+          nextHeight = lerp(
+            currentHeight,
+            smoothTargetHeight,
+            clamp01(smoothingStrength * weight)
+          );
+          break;
+        }
+        case "flatten":
+          if (referenceHeight === null || !Number.isFinite(referenceHeight)) {
+            throw new Error(
+              "Flatten terrain brush stamps require a finite reference height."
+            );
+          }
+
+          nextHeight = lerp(
+            currentHeight,
+            referenceHeight,
+            clamp01(smoothingStrength * weight)
+          );
+          break;
+      }
+
+      if (nextHeight !== currentHeight) {
+        nextHeights[sampleIndex] = nextHeight;
+        changed = true;
+      }
+    }
+  }
+
+  return changed ? createTerrain({ ...terrain, heights: nextHeights }) : terrain;
+}
+
+export function getTerrainBrushStrokeSpacing(
+  terrain: Terrain,
+  settings: TerrainBrushSettings
+): number {
+  return Math.max(terrain.cellSize * 0.5, settings.radius * 0.25);
+}