The 22-year-old centre back arrived at the Keepmoat Stadium in September and signed an initial short-term deal after being released by Arsenal.
He had spent 13 years at the Gunners but never made a senior appearance at the Emirates Stadium prior to his departure.
Securing Olowu’s services for an extended period was one of the club's main targets on the recruitment front.
Doncaster legend James Coppinger – who plays a role in the recruitment team – said: “We know the attributes a Doncaster Rovers centre back should have, and Joseph’s strong character, willingness to improve, and his performances on the pitch make him someone that we believe can have a bright future with this club.
“We don’t want to be a club that is always rebuilding – we have a clear idea of what a Doncaster Rovers team should look like and I’m working closely with the manager, Graham [Younger, talent identification manager] and the board so we can build something that everyone associated with the club can be proud of.”
Olowu has made 11 appearances for Rovers this season, as manager Richie Wellens added: “We believe we’ve got a great young centre back in Joseph. He has all the attributes to go really far in the game and we believe he could develop into a key player for us.
“His attitude on and off the pitch has been first class since he arrived, and we’re delighted that he sees his future here with us.”