diff --git a/SPEC.md b/SPEC.md
index 2689bd3..48ece35 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -376,15 +376,21 @@ Items are popped in reverse order (item1 is array[0]).
 Index is coerced to number and floored.
 
 #### ARRAY_SET
-**Operand**: None  
-**Effect**: Set array element at index (mutates array)  
-**Stack**: [array, index, value] → []  
+**Operand**: None
+**Effect**: Set array element at index (mutates array)
+**Stack**: [array, index, value] → []
 **Errors**: Throws if not array or index out of bounds
 
+#### ARRAY_PUSH
+**Operand**: None
+**Effect**: Append value to end of array (mutates array, grows by 1)
+**Stack**: [array, value] → []
+**Errors**: Throws if not array
+
 #### ARRAY_LEN
-**Operand**: None  
-**Effect**: Get array length  
-**Stack**: [array] → [length]  
+**Operand**: None
+**Effect**: Get array length
+**Stack**: [array] → [length]
 **Errors**: Throws if not array
 
 ### Dictionary Operations
diff --git a/src/opcode.ts b/src/opcode.ts
index 8d9f645..a48f436 100644
--- a/src/opcode.ts
+++ b/src/opcode.ts
@@ -48,6 +48,7 @@ export enum OpCode {
   MAKE_ARRAY,
   ARRAY_GET,
   ARRAY_SET,
+  ARRAY_PUSH,
   ARRAY_LEN,
 
   // dicts
diff --git a/src/vm.ts b/src/vm.ts
index 1c256fb..bbc12ff 100644
--- a/src/vm.ts
+++ b/src/vm.ts
@@ -212,6 +212,16 @@ export class VM {
         setArray.value[setIdx] = setValue
         break
 
+      case OpCode.ARRAY_PUSH:
+        const pushValue = this.stack.pop()!
+        const pushArray = this.stack.pop()!
+
+        if (pushArray.type !== 'array')
+          throw new Error('ARRAY_PUSH: not an array')
+
+        pushArray.value.push(pushValue)
+        break
+
       case OpCode.ARRAY_LEN:
         const lenArray = this.stack.pop()!
         if (lenArray.type !== 'array')
diff --git a/tests/basic.test.ts b/tests/basic.test.ts
index 26bb477..2ab8144 100644
--- a/tests/basic.test.ts
+++ b/tests/basic.test.ts
@@ -418,6 +418,33 @@ test("ARRAY_SET - sets element", async () => {
   expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 99 })
 })
 
+test("ARRAY_PUSH - appends to array", async () => {
+  const str = `
+  PUSH 10
+  PUSH 20
+  MAKE_ARRAY 2
+  DUP
+  PUSH 30
+  ARRAY_PUSH
+  ARRAY_LEN
+`
+  expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 3 })
+})
+
+test("ARRAY_PUSH - mutates original array", async () => {
+  const str = `
+  PUSH 10
+  PUSH 20
+  MAKE_ARRAY 2
+  DUP
+  PUSH 30
+  ARRAY_PUSH
+  PUSH 2
+  ARRAY_GET
+`
+  expect(await run(toBytecode(str))).toEqual({ type: 'number', value: 30 })
+})
+
 test("ARRAY_LEN - gets length", async () => {
   const str = `
   PUSH 10