Concise News understands that the deal is subject to worldwide clearance.
The 21-year-old - Steven Gerrard's ninth summer signing - impressed for NAC Breda last season, scoring five goals in 12 matches for the Dutch outfit. I got a phone call from my agent and he told me about Rangers and I said let's see what happens.
"Gerrard told me about the team, the players, he said everyone will help me if I came here and he will help me go forward in my career, and you don't have to think twice when someone says that to you". Even if the plan was to stay at Roma I couldn't turn him down.
Regrets mixed with pride as England fans digest World Cup heartbreak
Asked how far he thought the still young England team could progress in the next World Cup, he said: "This far, if not the final".
"I'm very happy to be here", Coulibaly, capped nine times for Mali, said.
"He (Gerrard) can help me to develop my football". I don't watch the league, but I know about Celtic and Rangers in Scotland.
"I spoke to him when I was in Mali and he asked me if I wanted to join his team, and I said yes, although I was anxious it wasn't actually Gerrard (on the phone)!"
He follows the signings of centre-backs Connor Goldson and Nikola Katic, right-back Jon Flanagan, wingers Jamie Murphy and Scott Arfield, goalkeeper Allan McGregor and midfielder Ovie Ejaria (loan).