Fournier, 28, is averaging a career-high 19.7 points and 3.7 assists per game this season. He is attempting a career-best 7.2 3-pointers per contest and making 38.8% of them.
"Evan has established himself as a highly-efficient scorer, whose combination of shooting and playmaking will make an immediate impact on this team," Celtics president Danny Ainge said in a statement. "He's someone who has consistently produced on the offensive end, and we think his unique abilities will fit in seamlessly with our current group."
For his nine-year career, Fournier is averaging 14.4 points per game. He gives the Celtics a solid scoring option off the bench.
The Celtics were able to take on Fournier's salary by using part of the trade exception that they created last year in the move that sent Gordon Hayward to Charlotte, according to the Associated Press.
Fournier is an unrestricted free agent after this season.
The Celtics started Thursday eighth in the Eastern Conference with a 21-23 record, well below their expectations coming into the season.
Information from The Associated Press was used in this report.