diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2b07680..7a0ff59 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 # Changelog
 
+- TypeScript definition for `ecfToEci` function is added. 
+
 ## 4.1.3 (2020-12-18)
 
 - Fixed calculation of Doppler effect (`dopplerFactor`).
diff --git a/types/index.d.ts b/types/index.d.ts
index aabe99a..41f2475 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -173,6 +173,11 @@ declare module 'satellite.js' {
    */
   export function eciToEcf<T>(positionEci: EciVec3<T>, gmst: GMSTime): EcfVec3<T>;
 
+  /**
+   * Convert ECF to ECI. Units are not modified.
+   */
+  export function ecfToEci<T>(positionEcf: EcfVec3<T>, gmst: GMSTime): EciVec3<T>;
+
   /**
    * Convert geodetic location to ECF
    */