diff --git a/hashes/sha3.py b/hashes/sha3.py
new file mode 100644
index 000000000000..cf99e263c811
--- /dev/null
+++ b/hashes/sha3.py
@@ -0,0 +1,164 @@
+"""
+Pure Python SHA-3 (Keccak-f[1600]) implementation
+
+Usage:
+  python sha3.py --string "hello"
+  python sha3.py --file data.bin
+"""
+
+import argparse
+import struct
+from typing import ClassVar
+
+
+class KeccakSHA3:
+    # Round constants
+    _RC: ClassVar[list[int]] = [
+        0x0000000000000001,
+        0x0000000000008082,
+        0x800000000000808A,
+        0x8000000080008000,
+        0x000000000000808B,
+        0x0000000080000001,
+        0x8000000080008081,
+        0x8000000000008009,
+        0x000000000000008A,
+        0x0000000000000088,
+        0x0000000080008009,
+        0x000000008000000A,
+        0x000000008000808B,
+        0x800000000000008B,
+        0x8000000000008089,
+        0x8000000000008003,
+        0x8000000000008002,
+        0x8000000000000080,
+        0x000000000000800A,
+        0x800000008000000A,
+        0x8000000080008081,
+        0x8000000000008080,
+        0x0000000080000001,
+        0x8000000080008008,
+    ]
+
+    _ROT: ClassVar[list[list[int]]] = [
+        [0, 36, 3, 41, 18],
+        [1, 44, 10, 45, 2],
+        [62, 6, 43, 15, 61],
+        [28, 55, 25, 21, 56],
+        [27, 20, 39, 8, 14],
+    ]
+
+    def __init__(self, message: bytes, bits: int = 256):
+        if bits not in (224, 256, 384, 512):
+            raise ValueError("Invalid SHA3 length")
+
+        self.msg = message
+        self.out_bits = bits
+        self.rate = 1600 - 2 * bits
+        self.state = [[0] * 5 for _ in range(5)]
+
+        self._absorb()
+        self.digest = self._squeeze().hex()
+
+    # ================= CORE =================
+
+    @staticmethod
+    def _rol(x: int, n: int) -> int:
+        n %= 64
+        return ((x << n) | (x >> (64 - n))) & 0xFFFFFFFFFFFFFFFF
+
+    def _permute(self) -> None:
+        a = self.state
+
+        for rnd in range(24):
+            # theta
+            c = [a[x][0] ^ a[x][1] ^ a[x][2] ^ a[x][3] ^ a[x][4] for x in range(5)]
+            d = [c[(x - 1) % 5] ^ self._rol(c[(x + 1) % 5], 1) for x in range(5)]
+
+            for x in range(5):
+                for y in range(5):
+                    a[x][y] ^= d[x]
+
+            # rho + pi
+            b = [[0] * 5 for _ in range(5)]
+            for x in range(5):
+                for y in range(5):
+                    b[y][(2 * x + 3 * y) % 5] = self._rol(a[x][y], self._ROT[x][y])
+
+            # chi
+            for x in range(5):
+                for y in range(5):
+                    a[x][y] = b[x][y] ^ ((~b[(x + 1) % 5][y]) & b[(x + 2) % 5][y])
+
+            # iota
+            a[0][0] ^= self._RC[rnd]
+
+    # ================= SPONGE =================
+
+    def _pad(self, data: bytes) -> bytes:
+        rate_bytes = self.rate // 8
+        buf = bytearray(data)
+        buf.append(0x06)
+        while len(buf) % rate_bytes != rate_bytes - 1:
+            buf.append(0x00)
+        buf.append(0x80)
+        return bytes(buf)
+
+    def _absorb(self) -> None:
+        rate_bytes = self.rate // 8
+        padded = self._pad(self.msg)
+
+        for off in range(0, len(padded), rate_bytes):
+            block = padded[off : off + rate_bytes]
+            for i in range(0, rate_bytes, 8):
+                lane = struct.unpack("<Q", block[i : i + 8])[0]
+                x = (i // 8) % 5
+                y = (i // 8) // 5
+                self.state[x][y] ^= lane
+            self._permute()
+
+    def _squeeze(self) -> bytes:
+        out = bytearray()
+        rate_bytes = self.rate // 8
+        need = self.out_bits // 8
+
+        while len(out) < need:
+            for i in range(0, rate_bytes, 8):
+                x = (i // 8) % 5
+                y = (i // 8) // 5
+                out.extend(struct.pack("<Q", self.state[x][y]))
+            if len(out) < need:
+                self._permute()
+
+        return bytes(out[:need])
+
+
+# ================= CLI =================
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description="SHA-3 hashing tool")
+    parser.add_argument("-s", "--string", help="String input")
+    parser.add_argument("-f", "--file", help="File input")
+    parser.add_argument(
+        "-l",
+        "--length",
+        type=int,
+        default=256,
+        choices=[224, 256, 384, 512],
+    )
+
+    args = parser.parse_args()
+
+    if args.file:
+        with open(args.file, "rb") as f:
+            data = f.read()
+    else:
+        data = (args.string or "Hello World").encode()
+
+    h = KeccakSHA3(data, args.length)
+    print(f"SHA3-{args.length}: {h.digest}")
+
+
+if __name__ == "__main__":
+    main()