The Nigerian forward was born in Kaduna, has struggled to pin down a regular shirt since linking up with AS Roma from Serie B side Spezia in 2015.
He has scored twice in six appearances for the Giallorossi since the move.
The striker has spent the majority of his Roma career out on loan and has enjoyed spells at Bologna and Torino, as well as Dutch side NAC Breda.
“The plan was to stay in Rome this season but when
Steven Gerrard calls, you can’t say no,” Sadiq told Rangers TV.
“He told me about the team, the players, that everyone will be at my side if I come here and he will help me to go forward in my career.
“You don’t have to think twice when someone says that to you.”
Sadiq was part of Nigeria’s U-23 team that won bronze medal in the football event at the Rio 2016 Olympics.
— Rangers Football Club (@RangersFC) July 10, 2018