The 25-year-old striker has played for three promotion-winning Championship sides in four seasons, at Hamilton, Hearts and Hibs.
United face Morton in the Premiership play-off quarter-final first leg at Cappielow on Tuesday, 9 May.
"I am joining a fantastic club with a great reputation who are set up to be in the Premiership," Keatings said.
"I look forward to joining Dundee United. I have enjoyed playing at Tannadice, and my goal is to be scoring in the Premiership for United whether it is next season or beyond."
Keatings made 30 appearances for Championship winners Hibs this season, scoring eight goals.
Hibernian striker James Keatings has signed a pre-contract agreement with Dundee United.