diff --git a/library/compiler-builtins/libm/src/lib.rs b/library/compiler-builtins/libm/src/lib.rs
index 0e1ed61d2776..1837b9c1f9ed 100644
--- a/library/compiler-builtins/libm/src/lib.rs
+++ b/library/compiler-builtins/libm/src/lib.rs
@@ -376,7 +376,6 @@ pub trait F64Ext: private::Sealed + Sized {
 
     fn atan(self) -> Self;
 
-    #[cfg(todo)]
     fn atan2(self, other: Self) -> Self;
 
     #[inline]
@@ -545,7 +544,6 @@ impl F64Ext for f64 {
         atan(self)
     }
 
-    #[cfg(todo)]
     #[inline]
     fn atan2(self, other: Self) -> Self {
         atan2(self, other)
diff --git a/library/compiler-builtins/libm/src/math/atan2.rs b/library/compiler-builtins/libm/src/math/atan2.rs
new file mode 100644
index 000000000000..324b1e937f94
--- /dev/null
+++ b/library/compiler-builtins/libm/src/math/atan2.rs
@@ -0,0 +1,72 @@
+use super::atan;
+use super::fabs;
+
+const PI: f64 = 3.1415926535897931160E+00; /* 0x400921FB, 0x54442D18 */
+const PI_LO: f64 = 1.2246467991473531772E-16; /* 0x3CA1A626, 0x33145C07 */
+
+#[inline]
+pub fn atan2(y: f64, x: f64) -> f64 {
+    if x.is_nan() || y.is_nan() {
+        return x + y;
+    }
+    let mut ix = (x.to_bits() >> 32) as u32;
+    let lx = x.to_bits() as u32;
+    let mut iy = (y.to_bits() >> 32) as u32;
+    let ly = y.to_bits() as u32;
+    if (ix - 0x3ff00000 | lx) == 0 {
+        /* x = 1.0 */
+        return atan(y);
+    }
+    let m = ((iy >> 31) & 1) | ((ix >> 30) & 2); /* 2*sign(x)+sign(y) */
+    ix &= 0x7fffffff;
+    iy &= 0x7fffffff;
+
+    /* when y = 0 */
+    if (iy | ly) == 0 {
+        return match m {
+            0 | 1 => y, /* atan(+-0,+anything)=+-0 */
+            2 => PI,    /* atan(+0,-anything) = PI */
+            _ => -PI,   /* atan(-0,-anything) =-PI */
+        };
+    }
+    /* when x = 0 */
+    if (ix | lx) == 0 {
+        return if m & 1 != 0 { -PI / 2.0 } else { PI / 2.0 };
+    }
+    /* when x is INF */
+    if ix == 0x7ff00000 {
+        if iy == 0x7ff00000 {
+            return match m {
+                0 => PI / 4.0,        /* atan(+INF,+INF) */
+                1 => -PI / 4.0,       /* atan(-INF,+INF) */
+                2 => 3.0 * PI / 4.0,  /* atan(+INF,-INF) */
+                _ => -3.0 * PI / 4.0, /* atan(-INF,-INF) */
+            };
+        } else {
+            return match m {
+                0 => 0.0,  /* atan(+...,+INF) */
+                1 => -0.0, /* atan(-...,+INF) */
+                2 => PI,   /* atan(+...,-INF) */
+                _ => -PI,  /* atan(-...,-INF) */
+            };
+        }
+    }
+    /* |y/x| > 0x1p64 */
+    if ix + (64 << 20) < iy || iy == 0x7ff00000 {
+        return if m & 1 != 0 { -PI / 2.0 } else { PI / 2.0 };
+    }
+
+    /* z = atan(|y/x|) without spurious underflow */
+    let z = if (m & 2 != 0) && iy + (64 << 20) < ix {
+        /* |y/x| < 0x1p-64, x<0 */
+        0.0
+    } else {
+        atan(fabs(y / x))
+    };
+    match m {
+        0 => z,                /* atan(+,+) */
+        1 => -z,               /* atan(-,+) */
+        2 => PI - (z - PI_LO), /* atan(+,-) */
+        _ => (z - PI_LO) - PI, /* atan(-,-) */
+    }
+}
diff --git a/library/compiler-builtins/libm/src/math/mod.rs b/library/compiler-builtins/libm/src/math/mod.rs
index 64ab3faa703c..754efa9f6dac 100644
--- a/library/compiler-builtins/libm/src/math/mod.rs
+++ b/library/compiler-builtins/libm/src/math/mod.rs
@@ -12,6 +12,7 @@ mod acosf;
 mod asin;
 mod asinf;
 mod atan;
+mod atan2;
 mod atan2f;
 mod atanf;
 mod cbrt;
@@ -72,6 +73,7 @@ pub use self::acosf::acosf;
 pub use self::asin::asin;
 pub use self::asinf::asinf;
 pub use self::atan::atan;
+pub use self::atan2::atan2;
 pub use self::atan2f::atan2f;
 pub use self::atanf::atanf;
 pub use self::cbrt::cbrt;
diff --git a/library/compiler-builtins/libm/test-generator/src/main.rs b/library/compiler-builtins/libm/test-generator/src/main.rs
index a3efd076962e..09a9cb8f5887 100644
--- a/library/compiler-builtins/libm/test-generator/src/main.rs
+++ b/library/compiler-builtins/libm/test-generator/src/main.rs
@@ -770,7 +770,7 @@ f64_f64! {
 
 // With signature `fn(f64, f64) -> f64`
 f64f64_f64! {
-    // atan2,
+    atan2,
     fdim,
     fmod,
     hypot,