The 25-year-old scored eight goals this season as Hibs won the Championship and had an offer to stay at Easter Road.
However, with 14 of his 30 appearances coming off the bench this term, Keatings has opted to move on in a bid for more regular starts.
"I look forward to joining Dundee United. I have enjoyed playing at Tannadice, and I know I am joining a fantastic club with a great reputation who are set up to be in the Premiership," he told United's website.
"My goal is to be scoring in the Premiership for United whether it is next season or beyond."
Dundee United finished third in the Championship this season and will have to contest the playoffs in a bid to reach the Scottish Premiership.