diff --git a/.changeset/fix-hex-signed-odd-length.md b/.changeset/fix-hex-signed-odd-length.md
new file mode 100644
index 00000000..6d11efa7
--- /dev/null
+++ b/.changeset/fix-hex-signed-odd-length.md
@@ -0,0 +1,5 @@
+---
+"ox": patch
+---
+
+Fixed `Hex.toBigInt` and `Hex.toNumber` throwing on odd-length hex with `signed: true`, and `Hex.fromNumber` returning `0x0` for a negative number with `signed: true` and no `size`.
diff --git a/src/core/Hex.ts b/src/core/Hex.ts
index 0253f0e0..a8683edf 100644
--- a/src/core/Hex.ts
+++ b/src/core/Hex.ts
@@ -286,7 +286,9 @@ export function fromNumber(
     maxValue = BigInt(Number.MAX_SAFE_INTEGER)
   }
 
-  const minValue = typeof maxValue === 'bigint' && signed ? -maxValue - 1n : 0
+  // Negative values need a `size` to encode as two's complement.
+  const minValue =
+    size && typeof maxValue === 'bigint' && signed ? -maxValue - 1n : 0
 
   if ((maxValue && value_ > maxValue) || value_ < minValue) {
     const suffix = typeof value === 'bigint' ? 'n' : ''
@@ -611,9 +613,7 @@ export function toBigInt(hex: Hex, options: toBigInt.Options = {}): bigint {
   const value = BigInt(hex)
   if (!signed) return value
 
-  const size = (hex.length - 2) / 2
-
-  const max_unsigned = (1n << (BigInt(size) * 8n)) - 1n
+  const max_unsigned = (1n << (BigInt(size(hex)) * 8n)) - 1n
   const max_signed = max_unsigned >> 1n
 
   if (value <= max_signed) return value
diff --git a/src/core/_test/Hex.test.ts b/src/core/_test/Hex.test.ts
index 184e89c7..9ccc32c8 100644
--- a/src/core/_test/Hex.test.ts
+++ b/src/core/_test/Hex.test.ts
@@ -221,6 +221,12 @@ describe('fromNumber', () => {
     ).toThrowErrorMatchingInlineSnapshot(
       '[Hex.IntegerOutOfRangeError: Number `-32769` is not in safe 16-bit signed integer range (`-32768` to `32767`)]',
     )
+    expect(() =>
+      // @ts-expect-error `signed` requires `size`.
+      Hex.fromNumber(-1, { signed: true }),
+    ).toThrowErrorMatchingInlineSnapshot(
+      '[Hex.IntegerOutOfRangeError: Number `-1` is not in safe signed integer range (`0` to `9007199254740991`)]',
+    )
   })
 })
 
@@ -404,6 +410,10 @@ describe('toBigInt', () => {
     expect(Hex.toBigInt('0x7fff', { signed: true })).toBe(32767n)
     expect(Hex.toBigInt('0x8000', { signed: true })).toBe(-32768n)
 
+    expect(Hex.toBigInt('0xf', { signed: true })).toBe(15n)
+    expect(Hex.toBigInt('0xfff', { signed: true })).toBe(4095n)
+    expect(Hex.toBigInt(Hex.fromNumber(420), { signed: true })).toBe(420n)
+
     expect(
       Hex.toBigInt(
         '0x000000000000000000000000000000000000000000000000aade1ed08b0b325c',