The 31-year-old joined up with his new team-mates at Rockliffe on Thursday morning after completing the formalities of his move to Teesside.
Bolasie brings a wealth of experience, having clocked up over 450 career appearances, including nine years as a Premier League player.
The DR Congo international – he has scored nine goals in 40 appearances for his country – enjoyed the most successful spell of his career at Crystal Palace, where he made almost 150 appearances and played under current Boro boss Neil Warnock.
The Boro manager says his arrival is a big boost.
“I’ve known Yannick for many years,” he said. “I asked him if he could help us out until the end of the season and get some fitness in at the same time.
“We can’t expect wonders but he’s a good lad and he can play different positions for me. He’ll be good in the dressing room and it’s a big boost for us.”