diff --git a/src/document/migrate-scene-document.ts b/src/document/migrate-scene-document.ts
index 3fa6832e..bb9d0d97 100644
--- a/src/document/migrate-scene-document.ts
+++ b/src/document/migrate-scene-document.ts
@@ -1,5 +1,8 @@
 import {
+  MATERIAL_TEXTURE_SLOTS,
+  createEmptyCustomMaterialTextureRefs,
   createStarterMaterialRegistry,
+  type CustomMaterialTextureRefs,
   type MaterialDef
 } from "../materials/starter-material-library";
 import type { Vec3 } from "../core/vector";
@@ -263,6 +266,7 @@ import {
   SPLINE_PATH_SCENE_DOCUMENT_VERSION,
   WHITEBOX_BOX_LIGHT_VOLUME_SCENE_DOCUMENT_VERSION,
   CELESTIAL_BODY_OVERLAY_SCENE_DOCUMENT_VERSION,
+  CUSTOM_PBR_MATERIALS_SCENE_DOCUMENT_VERSION,
   SCENE_DOCUMENT_VERSION,
   STATIC_SIMPLE_MODEL_COLLIDERS_SCENE_DOCUMENT_VERSION,
   TRIGGER_ACTION_TARGET_FOUNDATION_SCENE_DOCUMENT_VERSION,
diff --git a/src/document/scene-document.ts b/src/document/scene-document.ts
index 2c511067..5b6d4a2a 100644
--- a/src/document/scene-document.ts
+++ b/src/document/scene-document.ts
@@ -36,7 +36,8 @@ import {
   type FoliagePrototypeRegistry
 } from "../foliage/foliage";
 
-export const SCENE_DOCUMENT_VERSION = 109 as const;
+export const SCENE_DOCUMENT_VERSION = 110 as const;
+export const CUSTOM_PBR_MATERIALS_SCENE_DOCUMENT_VERSION = 110 as const;
 export const SPLINE_CORRIDOR_JUNCTION_SHAPE_SCENE_DOCUMENT_VERSION = 109 as const;
 export const SPLINE_CORRIDOR_JUNCTION_EDGE_SCENE_DOCUMENT_VERSION = 108 as const;
 export const SPLINE_CORRIDOR_JUNCTIONS_SCENE_DOCUMENT_VERSION = 107 as const;
diff --git a/src/materials/starter-material-library.ts b/src/materials/starter-material-library.ts
index 58521072..a4bf13ef 100644
--- a/src/materials/starter-material-library.ts
+++ b/src/materials/starter-material-library.ts
@@ -564,35 +564,46 @@ function cloneMaterialSizeCm(sizeCm: MaterialSizeCm): MaterialSizeCm {
   };
 }
 
-export const STARTER_MATERIAL_LIBRARY: readonly MaterialDef[] =
+export const STARTER_MATERIAL_LIBRARY: readonly StarterMaterialDef[] =
   STARTER_MATERIAL_CATALOG.map((entry) => ({
     ...entry,
+    kind: "starter",
     sizeCm: cloneMaterialSizeCm(entry.sizeCm),
     tags: createMaterialTags(entry)
   }));
 
-export function getStarterMaterialAssetDirectory(material: MaterialDef): string {
+export function getStarterMaterialAssetDirectory(
+  material: StarterMaterialDef
+): string {
   return `${STARTER_MATERIAL_ASSET_ROOT}/${material.assetFolder}`;
 }
 
-export function getStarterMaterialPreviewUrl(material: MaterialDef): string {
+export function getStarterMaterialPreviewUrl(
+  material: StarterMaterialDef
+): string {
   return `${getStarterMaterialAssetDirectory(material)}/${material.previewImageName}`;
 }
 
-export function getStarterMaterialBaseColorUrl(material: MaterialDef): string {
+export function getStarterMaterialBaseColorUrl(
+  material: StarterMaterialDef
+): string {
   return `${getStarterMaterialAssetDirectory(material)}/basecolor.webp`;
 }
 
-export function getStarterMaterialNormalUrl(material: MaterialDef): string {
+export function getStarterMaterialNormalUrl(
+  material: StarterMaterialDef
+): string {
   return `${getStarterMaterialAssetDirectory(material)}/normal.webp`;
 }
 
-export function getStarterMaterialRoughnessUrl(material: MaterialDef): string {
+export function getStarterMaterialRoughnessUrl(
+  material: StarterMaterialDef
+): string {
   return `${getStarterMaterialAssetDirectory(material)}/roughness.webp`;
 }
 
 export function getStarterMaterialMetallicUrl(
-  material: MaterialDef
+  material: StarterMaterialDef
 ): string | null {
   return material.workflow === "metallic-roughness"
     ? `${getStarterMaterialAssetDirectory(material)}/metallic.webp`
@@ -600,14 +611,14 @@ export function getStarterMaterialMetallicUrl(
 }
 
 export function getStarterMaterialSpecularUrl(
-  material: MaterialDef
+  material: StarterMaterialDef
 ): string | null {
   return material.workflow === "specular-roughness"
     ? `${getStarterMaterialAssetDirectory(material)}/specular.webp`
     : null;
 }
 
-export function getStarterMaterialTileSizeMeters(material: MaterialDef): {
+export function getStarterMaterialTileSizeMeters(material: StarterMaterialDef): {
   x: number;
   y: number;
 } {
@@ -617,7 +628,7 @@ export function getStarterMaterialTileSizeMeters(material: MaterialDef): {
   };
 }
 
-export function getStarterMaterialTextureRepeat(material: MaterialDef): {
+export function getStarterMaterialTextureRepeat(material: StarterMaterialDef): {
   x: number;
   y: number;
 } {
@@ -630,10 +641,18 @@ export function getStarterMaterialTextureRepeat(material: MaterialDef): {
 }
 
 export function cloneMaterialDef(material: MaterialDef): MaterialDef {
+  if (material.kind === "starter") {
+    return {
+      ...material,
+      sizeCm: cloneMaterialSizeCm(material.sizeCm),
+      tags: [...material.tags]
+    };
+  }
+
   return {
     ...material,
-    sizeCm: cloneMaterialSizeCm(material.sizeCm),
-    tags: [...material.tags]
+    tags: [...material.tags],
+    textures: cloneCustomMaterialTextureRefs(material.textures)
   };
 }
 
@@ -656,3 +675,68 @@ export function createStarterMaterialRegistry(): Record<string, MaterialDef> {
     ])
   );
 }
+
+export function createEmptyCustomMaterialTextureRefs(): CustomMaterialTextureRefs {
+  return {
+    albedo: null,
+    normal: null,
+    roughness: null,
+    metallic: null
+  };
+}
+
+export function cloneCustomMaterialTextureRefs(
+  textures: CustomMaterialTextureRefs
+): CustomMaterialTextureRefs {
+  return {
+    albedo:
+      textures.albedo === null ? null : { assetId: textures.albedo.assetId },
+    normal:
+      textures.normal === null ? null : { assetId: textures.normal.assetId },
+    roughness:
+      textures.roughness === null
+        ? null
+        : { assetId: textures.roughness.assetId },
+    metallic:
+      textures.metallic === null
+        ? null
+        : { assetId: textures.metallic.assetId }
+  };
+}
+
+export function createCustomMaterialDef(
+  overrides: Partial<
+    Pick<
+      CustomMaterialDef,
+      | "id"
+      | "name"
+      | "swatchColorHex"
+      | "tags"
+      | "albedoColorHex"
+      | "opacity"
+      | "roughness"
+      | "metallic"
+      | "normalStrength"
+      | "textures"
+    >
+  > = {}
+): CustomMaterialDef {
+  const albedoColorHex = overrides.albedoColorHex ?? "#ffffff";
+
+  return {
+    id: overrides.id ?? createOpaqueId("material"),
+    kind: "custom",
+    name: overrides.name ?? "Custom Material",
+    swatchColorHex: overrides.swatchColorHex ?? albedoColorHex,
+    tags: overrides.tags ?? ["custom"],
+    albedoColorHex,
+    opacity: overrides.opacity ?? 1,
+    roughness: overrides.roughness ?? 0.8,
+    metallic: overrides.metallic ?? 0,
+    normalStrength: overrides.normalStrength ?? 1,
+    textures:
+      overrides.textures === undefined
+        ? createEmptyCustomMaterialTextureRefs()
+        : cloneCustomMaterialTextureRefs(overrides.textures)
+  };
+}
diff --git a/src/materials/starter-material-textures.ts b/src/materials/starter-material-textures.ts
index afd6dea6..742cb634 100644
--- a/src/materials/starter-material-textures.ts
+++ b/src/materials/starter-material-textures.ts
@@ -12,7 +12,7 @@ import {
   getStarterMaterialRoughnessUrl,
   getStarterMaterialSpecularUrl,
   getStarterMaterialTextureRepeat,
-  type MaterialDef
+  type StarterMaterialDef
 } from "./starter-material-library";
 
 export interface StarterMaterialTextureSet {
@@ -23,8 +23,11 @@ export interface StarterMaterialTextureSet {
   specular: Texture | null;
 }
 
-export function createStarterMaterialSignature(material: MaterialDef): string {
+export function createStarterMaterialSignature(
+  material: StarterMaterialDef
+): string {
   return [
+    material.kind,
     material.assetFolder,
     material.workflow,
     material.previewImageName,
@@ -72,7 +75,7 @@ function createRepeatableTextureImage(
 
 function configureTexture(
   texture: Texture,
-  material: MaterialDef,
+  material: StarterMaterialDef,
   options: { colorSpace?: typeof SRGBColorSpace | null }
 ) {
   const repeat = getStarterMaterialTextureRepeat(material);
@@ -85,7 +88,7 @@ function configureTexture(
 function loadMaterialTexture(
   loader: TextureLoader,
   url: string,
-  material: MaterialDef,
+  material: StarterMaterialDef,
   options: { colorSpace?: typeof SRGBColorSpace | null }
 ): Texture {
   const texture = loader.load(url, (loadedTexture) => {
@@ -102,7 +105,7 @@ function loadMaterialTexture(
 }
 
 export function createStarterMaterialTextureSet(
-  material: MaterialDef,
+  material: StarterMaterialDef,
   loader: TextureLoader = new TextureLoader()
 ): StarterMaterialTextureSet {
   const metallicUrl = getStarterMaterialMetallicUrl(material);