The summer signing from Hibs will spend the rest of the season at Ibrox after scoring just four goals in 18 appearances for Forest, and could join on a permanent basis in the summer.
Signed by former Rangers manager Mark Warburton, Cummings failed to hold down a regular starting berth at the City Ground and was understood to have held talks with Warburton’s successor Aitor Karanka last week.
The 22-year-old underwent a medical in Glasgow at the weekend before finalising his move to Rangers on Monday, and is the fourth player to join the Gers this month after Sean Goss and Jamie Murphy arrived on loan and Declan John signed a permanent deal.
Cummings told RangersTV: “I’m absolutely delighted to come to such a massive club. Once I knew they were interested, there was no question in my mind that I wanted to sign for them.
“It’s such a massive club and it’s a privilege to be here. I can’t wait to get out there at Ibrox and wear the blue.
“I can’t wait to get started now. I asked my agent to try and sort something out, and when I heard Rangers were interested it was just something I couldn’t decline.
“I have always enjoyed playing against Rangers, and hopefully I can enjoy playing on the other side this time.”
Scotland international Russell Martin could become Graeme Murty’s fifth signing of the January transfer window, wth the Norwich defender expected to put pen to paper on a loan deal later this week.
Martin hasn’t featured for the Canaries this season, and knows Murty well from the Rangers manager’s time as a youth coach at Carrow Road.
Terms have reportedly been agreed between the two clubs and Martin will likely undergo a medical in Glasgow in the next 24 hours.