The 19-year-old, who spent three months on loan at Loftus Road earlier this season, has agreed a three-and-a-half year contract with the west Londoners.
"I'm so pleased to be back here. I really enjoyed my time here on loan earlier this season and when the club showed an interest in offering me a permanent deal, I was made up," Ephraim told Rangers' official website.
"It's a great time to be at QPR. Big things are going to happen here - the new owners are very ambitious."