diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2036b999..f8fba738 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 - `str.pad_start`,`str.pad_end`を追加
 - `arr.insert`,`arr.remove`を追加
 - `arr.sort`の処理を非同期的にして高速化
+- `arr.flat`,`arr.flat_map`を追加
 
 # 0.18.0
 - `Core:abort`でプログラムを緊急停止できるように
diff --git a/docs/primitive-props.md b/docs/primitive-props.md
index bf9087eb..6f66d498 100644
--- a/docs/primitive-props.md
+++ b/docs/primitive-props.md
@@ -196,6 +196,14 @@ _fromIndex_ および _toIndex_ に関する挙動は`arr.slice`に準拠しま
 `arr.copy`同様シャローコピーであり、配列やオブジェクトの参照は維持されます。  
 _times_ には0以上の整数値を指定します。それ以外ではエラーになります。  
 
+### @(_v_: arr).flat(_depth_?: num): arr
+配列に含まれる配列を _depth_ で指定した深さの階層まで結合した新しい配列を作成します。  
+_depth_ には0以上の整数値を指定します。省略時は1になります。  
+
+### @(_v_: arr).flat_map(_func_: @(_item_: value, _index_: num) { value }): arr
+配列の各要素を _func_ の返り値で置き換えた後、1階層平坦化した新しい配列を作成します。  
+_func_ は非同期的に呼び出されます。
+
 ### @(_v_: arr).insert(_index_: num, _item_: value): null
 **【この操作は配列を書き換えます】**  
 配列の _index_ の位置に _item_ を挿入します。\
diff --git a/src/interpreter/primitive-props.ts b/src/interpreter/primitive-props.ts
index 07b9a08e..bfc89e55 100644
--- a/src/interpreter/primitive-props.ts
+++ b/src/interpreter/primitive-props.ts
@@ -2,7 +2,7 @@
 import { substring, length, indexOf, toArray } from 'stringz';
 import { AiScriptRuntimeError } from '../error.js';
 import { textEncoder } from '../const.js';
-import { assertArray, assertBoolean, assertFunction, assertNumber, assertString, expectAny, eq } from './util.js';
+import { assertArray, assertBoolean, assertFunction, assertNumber, assertString, expectAny, eq, isArray } from './util.js';
 import { ARR, FALSE, FN_NATIVE, NULL, NUM, STR, TRUE } from './value.js';
 import type { Value, VArr, VFn, VNum, VStr, VError } from './value.js';
 
@@ -296,6 +296,39 @@ const PRIMITIVE_PROPS: {
 			}
 		}),
 
+		flat: (target: VArr): VFn => FN_NATIVE(async ([depth], opts) => {
+			depth = depth ?? NUM(1);
+			assertNumber(depth);
+			if (!Number.isInteger(depth.value)) throw new AiScriptRuntimeError('arr.flat expected integer, got non-integer');
+			if (depth.value < 0) throw new AiScriptRuntimeError('arr.flat expected non-negative number, got negative');
+			const flat = (arr: Value[], depth: number, result: Value[]) => {
+				if (depth === 0) {
+					result.push(...arr);
+					return;
+				}
+				for (const v of arr) {
+					if (isArray(v)) {
+						flat(v.value, depth - 1, result);
+					} else {
+						result.push(v);
+					}
+				}
+			};
+			const result: Value[] = [];
+			flat(target.value, depth.value, result);
+			return ARR(result);
+		}),
+
+		flat_map: (target: VArr): VFn => FN_NATIVE(async ([fn], opts) => {
+			assertFunction(fn);
+			const vals = target.value.map(async (item, i) => {
+				const result = await opts.call(fn, [item, NUM(i)]);
+				return isArray(result) ? result.value : result;
+			});
+			const mapped_vals = await Promise.all(vals);
+			return ARR(mapped_vals.flat());
+    }),
+
 		every: (target: VArr): VFn => FN_NATIVE(async ([fn], opts) => {
 			assertFunction(fn);
 			for (let i = 0; i < target.value.length; i++) {
diff --git a/test/index.ts b/test/index.ts
index 0a6bddb7..529e85a5 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -2931,6 +2931,49 @@ describe('primitive props', () => {
 			]));
 		});
 		
+		test.concurrent('flat', async () => {
+			const res = await exe(`
+				var arr1 = [0, [1], [2, 3], [4, [5, 6]]]
+				let arr2 = arr1.flat()
+				let arr3 = arr1.flat(2)
+				<: [arr1, arr2, arr3]
+			`);
+			eq(res, ARR([
+				ARR([
+					NUM(0), ARR([NUM(1)]), ARR([NUM(2), NUM(3)]),
+					ARR([NUM(4), ARR([NUM(5), NUM(6)])])
+				]), // target not changed
+				ARR([
+					NUM(0), NUM(1), NUM(2), NUM(3),
+					NUM(4), ARR([NUM(5), NUM(6)]),
+				]),
+				ARR([
+					NUM(0), NUM(1), NUM(2), NUM(3),
+					NUM(4), NUM(5), NUM(6),
+				]),
+			]));
+		});
+		
+		test.concurrent('flat_map', async () => {
+			const res = await exe(`
+				let arr1 = [0, 1, 2]
+				let arr2 = ["a", "b"]
+				let arr3 = arr1.flat_map(@(x){ arr2.map(@(y){ [x, y] }) })
+				<: [arr1, arr3]
+			`);
+			eq(res, ARR([
+				ARR([NUM(0), NUM(1), NUM(2)]), // target not changed
+				ARR([
+					ARR([NUM(0), STR("a")]),
+					ARR([NUM(0), STR("b")]),
+					ARR([NUM(1), STR("a")]),
+					ARR([NUM(1), STR("b")]),
+					ARR([NUM(2), STR("a")]),
+					ARR([NUM(2), STR("b")]),
+				]),
+      ]));
+    });
+
 		test.concurrent('every', async () => {
 			const res = await exe(`
 				let arr1 = [0, 1, 2, 3]