The Dutch-born Curacao internationalist moved to Ibrox from Huddersfield Town in the summer but has failed to make an impression, both under Steven Gerrard and his successor Giovanni van Bronckhorst, starting just three times in a total of 12 appearances.
His only goal for the Scottish champions came in a 4-2 win over Ross County on November 7.
The 24-year-old will now return to the English Championship with Birmingham after signing a three-and-a-half year deal with Lee Bowyer’s side.
A club statement read: “Rangers can confirm it has accepted an offer for an undisclosed fee from Birmingham City for the permanent transfer of Juninho Bacuna.
“The Curacao internationalist joined Gers back in August from Huddersfield Town and has since made 12 appearances and scored one goal.
“Everyone at Rangers wishes Juninho well for his move and future career.”