diff --git a/src/cis.rs b/src/cis.rs
new file mode 100644
index 0000000..108d75d
--- /dev/null
+++ b/src/cis.rs
@@ -0,0 +1,65 @@
+//! Trait-based `cis` for both real and complex inputs.
+//!
+//! This provides a `cis()` method for both real floats and `Complex<T>` so callers can
+//! uniformly write `x.cis()` whether `x` is a `T` (float) or a `Complex<T>`.
+
+/// Compute cis(x) = exp(i*x). Implemented for both real floats and Complex<T>.
+///
+/// - For real `T`, `x.cis()` returns `Complex::new(x.cos(), x.sin())`.
+/// - For `Complex<T>`, `z.cis()` returns `exp(i*z) = exp(-Im(z)) * cis(Re(z))`.
+pub trait Cis {
+    type Output;
+    fn cis(self) -> Self::Output;
+}
+
+#[cfg(any(feature = "std", feature = "libm"))]
+mod imp {
+    use super::Cis;
+    use crate::Complex;
+    use num_traits::Float;
+
+    impl<T: Float> Cis for T {
+        type Output = Complex<T>;
+
+        fn cis(self) -> Complex<T> {
+            // cis(x) = cos(x) + i*sin(x)
+            Complex::new(self.cos(), self.sin())
+        }
+    }
+
+    impl<T: Float> Cis for Complex<T> {
+        type Output = Complex<T>;
+
+        fn cis(self) -> Complex<T> {
+            // cis(a+ib) = exp(-b) * cis(a)
+            let scale = (-self.im).exp();
+            self.re.cis() * scale
+        }
+    }
+
+    #[cfg(test)]
+    mod tests {
+        use crate::Cis;
+        use crate::Complex;
+
+        #[test]
+        fn cis_real() {
+            // Compare a hard-coded example to a reference result via WolframAlpha:
+            // https://www.wolframalpha.com/input?i=cis%281.2345%29
+
+            let theta = 1.2345;
+            let expected = Complex::new(0.3299931576785677, 0.9439833239445111);
+            assert!((expected - theta.cis()).norm_sqr() < 1e-10);
+        }
+
+        #[test]
+        fn cis_complex() {
+            // Compare a hard-coded example to a reference result via WolframAlpha:
+            // https://www.wolframalpha.com/input?i=cis%28-6.789%2Bi1.2345%29
+
+            let z = Complex::new(-6.7890, 1.2345);
+            let expected = Complex::new(0.254543682056692, -0.1409858152159941);
+            assert!((expected - z.cis()).norm_sqr() < 1e-10);
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 661b67b..21966a8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -37,6 +37,8 @@ use num_traits::float::FloatCore;
 use num_traits::float::{Float, FloatConst};
 
 mod cast;
+mod cis;
+pub use cis::Cis;
 mod pow;
 
 #[cfg(any(feature = "std", feature = "libm"))]