Haynes-Brown arrived at the Abbey on loan in November and has since made four appearances for the U’s, three of which were as a substitute.
The 23-year-old left-back was recently released by Yeovil and has now put pen-to-paper on a contract running until the end of the season.
Speaking about his permanent move, the former AFC Sudbury and Lowestoft Town player said: “I am very pleased to have signed a contract with Cambridge United.
“During my loan spell the coaching staff and players made me feel welcome and a part of the Club, so that made my choice to stay permanently a lot easier.
“With my contract being until the end of the season I want to play as well as I can hopefully spend more of my future with Cambridge United.
“The facilities at the club are good, I am a big fan of Richard Money’s philosophies of the game and feel that he can help me progress as a footballer.”
