diff --git a/cranelift/codegen/src/ir/immediates.rs b/cranelift/codegen/src/ir/immediates.rs
index 4a8d5e1fbc..5c36f362ab 100644
--- a/cranelift/codegen/src/ir/immediates.rs
+++ b/cranelift/codegen/src/ir/immediates.rs
@@ -264,6 +264,74 @@ impl FromStr for Uimm32 {
     }
 }
 
+/// A 128-bit unsigned integer immediate operand.
+///
+/// This is used as an immediate value in SIMD instructions
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
+pub struct Uimm128(pub [u8; 16]);
+
+impl Display for Uimm128 {
+    // print a 128-bit vector in hexadecimal, e.g. 0x000102030405060708090a0b0c0d0e0f
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "0x")?;
+        let mut anything_written = false;
+        for &b in self.0.iter() {
+            if b == 0 && !anything_written {
+                continue;
+            } else {
+                anything_written = true;
+                write!(f, "{:02x}", b)?;
+            }
+        }
+        if !anything_written {
+            write!(f, "00")?;
+        }
+        Ok(())
+    }
+}
+
+impl From<u64> for Uimm128 {
+    fn from(x: u64) -> Self {
+        let mut buffer: [u8; 16] = [0; 16]; // zero-fill
+        (0..8).for_each(|byte| buffer[15 - byte] = (x >> (byte as u64 * 8) & 0xff) as u8); // insert each byte from the u64 into v after byte position 8
+        Uimm128(buffer)
+    }
+}
+
+impl From<&[u8]> for Uimm128 {
+    fn from(slice: &[u8]) -> Self {
+        assert_eq!(slice.len(), 16);
+        let mut buffer = [0; 16];
+        buffer.copy_from_slice(slice);
+        Uimm128(buffer)
+    }
+}
+
+impl FromStr for Uimm128 {
+    type Err = &'static str;
+
+    // parse a 128-bit vector from a hexadecimal string, formatted as above
+    fn from_str(s: &str) -> Result<Self, &'static str> {
+        if s.len() <= 2 || &s[0..2] != "0x" {
+            Err("Expected a hexadecimal string, e.g. 0x1234")
+        } else if s.len() % 2 != 0 {
+            Err("Hexadecimal string must have an even number of digits")
+        } else if s.len() > 32 {
+            Err("Hexadecimal string has too many digits to fit in a 128-bit vector")
+        } else {
+            let mut buffer = [0; 16]; // zero-fill
+            let start_at = 16 - s.len() / 2;
+            for i in (2..s.len()).step_by(2) {
+                let byte = u8::from_str_radix(&s[i..i + 2], 16)
+                    .or_else(|_| Err("Unable to parse as hexadecimal"))?;
+                let position = start_at + (i / 2);
+                buffer[position] = byte;
+            }
+            Ok(Uimm128(buffer))
+        }
+    }
+}
+
 /// 32-bit signed immediate offset.
 ///
 /// This is used to encode an immediate offset for load/store instructions. All supported ISAs have
@@ -884,6 +952,38 @@ mod tests {
         parse_err::<Uimm64>("0x0_0000_0000_0000_0000", "Too many hexadecimal digits");
     }
 
+    #[test]
+    fn format_uimm128() {
+        assert_eq!(Uimm128::from(0).to_string(), "0x00");
+        assert_eq!(Uimm128::from(42).to_string(), "0x2a");
+        assert_eq!(Uimm128::from(3735928559).to_string(), "0xdeadbeef");
+        assert_eq!(
+            Uimm128::from(0x0102030405060708).to_string(),
+            "0x0102030405060708"
+        );
+    }
+
+    #[test]
+    fn parse_uimm128() {
+        parse_ok::<Uimm128>("0x00", "0x00");
+        parse_ok::<Uimm128>("0x00000042", "0x42");
+        parse_ok::<Uimm128>(
+            "0x0102030405060708090a0b0c0d0e0f",
+            "0x0102030405060708090a0b0c0d0e0f",
+        );
+
+        parse_err::<Uimm128>("", "Expected a hexadecimal string, e.g. 0x1234");
+        parse_err::<Uimm128>("0x", "Expected a hexadecimal string, e.g. 0x1234");
+        parse_err::<Uimm128>(
+            "0x042",
+            "Hexadecimal string must have an even number of digits",
+        );
+        parse_err::<Uimm128>(
+            "0x00000000000000000000000000000000000000000000000000",
+            "Hexadecimal string has too many digits to fit in a 128-bit vector",
+        );
+    }
+
     #[test]
     fn format_offset32() {
         assert_eq!(Offset32(0).to_string(), "");