diff --git a/Userland/Libraries/LibM/math.cpp b/Userland/Libraries/LibM/math.cpp
index a17666af81..115e5f5200 100644
--- a/Userland/Libraries/LibM/math.cpp
+++ b/Userland/Libraries/LibM/math.cpp
@@ -742,4 +742,15 @@ long double nexttowardl(long double, long double) NOEXCEPT
 {
     TODO();
 }
+
+double copysign(double x, double y)
+{
+    if (x < 0 && y < 0)
+        return x;
+    if (x >= 0 && y < 0)
+        return -x;
+    if (x < 0 && y >= 0)
+        return -x;
+    return x;
+}
 }
diff --git a/Userland/Libraries/LibM/math.h b/Userland/Libraries/LibM/math.h
index a13acd8355..02e7c1f7a4 100644
--- a/Userland/Libraries/LibM/math.h
+++ b/Userland/Libraries/LibM/math.h
@@ -143,4 +143,6 @@ double nexttoward(double, long double) NOEXCEPT;
 float nexttowardf(float, long double) NOEXCEPT;
 long double nexttowardl(long double, long double) NOEXCEPT;
 
+double copysign(double x, double y);
+
 __END_DECLS