diff --git a/.changeset/bytearray-toi32-overflow.md b/.changeset/bytearray-toi32-overflow.md
new file mode 100644
index 000000000..c977b05ef
--- /dev/null
+++ b/.changeset/bytearray-toi32-overflow.md
@@ -0,0 +1,10 @@
+---
+'@graphprotocol/graph-ts': patch
+---
+
+`ByteArray.toI32` and `ByteArray.toI64` now report an overflow for a positive value
+that needs the sign bit of the widest byte, instead of returning it as a negative
+number. The overflow check only compared the bytes above the target width against the
+sign padding, so `0x80000000` carried in five bytes - exactly what
+`BigInt.fromUnsignedBytes` produces for a `u32` at or above `2^31` - passed the check
+and came back as `-2147483648`.
diff --git a/packages/ts/common/collections.ts b/packages/ts/common/collections.ts
index 706b92ec8..fa2b06184 100644
--- a/packages/ts/common/collections.ts
+++ b/packages/ts/common/collections.ts
@@ -150,6 +150,14 @@ export class ByteArray extends Uint8Array {
         assert(false, 'overflow converting ' + this.toHexString() + ' to i32');
       }
     }
+    // Matching sign padding above the window is not enough on its own: the sign bit of the
+    // last byte inside the window has to agree with it too. Without this check a positive
+    // value one bit too wide - 0x80000000 carried in five bytes, exactly what
+    // BigInt.fromUnsignedBytes produces for a u32 >= 2^31 - passes the loop above and is
+    // returned as a negative i32 instead of throwing.
+    if (this.length > 4 && this[3] >> 7 != (isNeg ? 1 : 0)) {
+      assert(false, 'overflow converting ' + this.toHexString() + ' to i32');
+    }
     const paddedBytes = new Bytes(4);
     paddedBytes[0] = padding;
     paddedBytes[1] = padding;
@@ -195,6 +203,11 @@ export class ByteArray extends Uint8Array {
         assert(false, 'overflow converting ' + this.toHexString() + ' to i64');
       }
     }
+    // Same as in toI32: the sign bit of the last byte inside the window has to agree with
+    // the padding, otherwise a positive value that needs the 64th bit is returned negative.
+    if (this.length > 8 && this[7] >> 7 != (isNeg ? 1 : 0)) {
+      assert(false, 'overflow converting ' + this.toHexString() + ' to i64');
+    }
     const paddedBytes = new Bytes(8);
     paddedBytes[0] = padding;
     paddedBytes[1] = padding;
diff --git a/packages/ts/test/bigInt.ts b/packages/ts/test/bigInt.ts
index 202dc1a0d..c5f5c1169 100644
--- a/packages/ts/test/bigInt.ts
+++ b/packages/ts/test/bigInt.ts
@@ -122,7 +122,9 @@ export function testBigInt(): void {
   longArray[3] = 255;
   longArray[4] = 0;
   assert(longArray.toU32() == 4_294_705_147);
-  assert(longArray.toI32() == 4_294_705_147);
+  // 4_294_705_147 is past i32.MAX_VALUE, so toI32 reports an overflow rather than
+  // returning the wrapped -262_149. The assert cannot be written here: an overflow
+  // aborts, and this harness has no way to expect one.
 
   const bytes = Bytes.fromHexString('0x56696b746f726961');
   assert((bytes[0] = 0x56));
diff --git a/packages/ts/test/bytes.ts b/packages/ts/test/bytes.ts
index f62004fed..850b0e564 100644
--- a/packages/ts/test/bytes.ts
+++ b/packages/ts/test/bytes.ts
@@ -9,7 +9,34 @@ export function testBytesWithByteArray(): void {
   longArray[3] = 255;
   longArray[4] = 0;
   assert(longArray.toU32() == 4_294_705_147);
-  assert(longArray.toI32() == 4_294_705_147);
+  // 4_294_705_147 is past i32.MAX_VALUE, so toI32 reports an overflow rather than
+  // returning the wrapped -262_149. The assert cannot be written here: an overflow
+  // aborts, and this harness has no way to expect one.
+
+  // Widening a value with sign padding must keep converting - these all still fit.
+  const i32Max = new ByteArray(5);
+  i32Max[0] = 255;
+  i32Max[1] = 255;
+  i32Max[2] = 255;
+  i32Max[3] = 127;
+  i32Max[4] = 0;
+  assert(i32Max.toI32() == 2_147_483_647);
+
+  const i32Min = new ByteArray(5);
+  i32Min[0] = 0;
+  i32Min[1] = 0;
+  i32Min[2] = 0;
+  i32Min[3] = 128;
+  i32Min[4] = 255;
+  assert(i32Min.toI32() == -2_147_483_648);
+
+  // The same value in exactly four bytes is a plain i32 and is unaffected.
+  const i32MinNarrow = new ByteArray(4);
+  i32MinNarrow[0] = 0;
+  i32MinNarrow[1] = 0;
+  i32MinNarrow[2] = 0;
+  i32MinNarrow[3] = 128;
+  assert(i32MinNarrow.toI32() == -2_147_483_648);
 
   const bytes = Bytes.fromHexString('0x56696b746f726961');
   assert((bytes[0] = 0x56));