The 22-year-old Mali midfielder is Gerrard's eighth summer signing.
Rangers have confirmed the signing of Roma striker Umar Sadiq on a season-long loan, pending global clearance.
"Rangers are delighted to announce the signing of Umar Sadiq on a season-long loan from Roma, subject to global clearance", read a statement on club website.
The 22-year-old midfielder is the eighth signing of the summer window for new manager Steven Gerrard, subject to worldwide clearance.
"I sat the young lads down on day one and told them they would come over to Spain and it was down to them to mix it with the senior players and let them know you're here to challenge them", he said. Even if the plan was to stay at Roma I couldn't turn him down.
"Rangers are a good team and I knew the name before Gerrard came in". I don't watch the league, but I know about Celtic and Rangers in Scotland. However, Liverpool legend has until midnight on Wednesday to add to players to his squad for the Europa League clash with Shkupi on Thursday.
"I am very happy to be here".
Mali worldwide Coulibaly, who has been capped 10 times, admits the lure of playing for Gerrard was too good to turn down.
Coulibaly follows veteran goalkeeper Allan McGregor, full-back Jon Flanagan, centre-backs Connor Goldson and Nikola Katic, midfielders Scott Arfield and Ovie Ejaria, and winger Jamie Murphy - on a permanent deal after a successful loan spell - in joining Rangers.
"He can help me to develop my football". He just asked me whether if I wanted to join his team or not and I said yes.