Unga, 26, signed with the Ravens as an undrafted free agent this past spring. The Midwestern State product spent the first 11 weeks of the season on the Ravens' practice squad.
Coach Doug Marrone said that Unga, who played tackle in college, will switch to guard with the Bills.
Unga stands at 6-foot-5 and 320 pounds. Marrone said Unga's size is part of what led to his signing.
"What we like to do is build a bigger team," Marrone said. "My philosophy has always been that this is a big man's game."
Here's a look at the size of the Bills' roster at offensive line:
Erik Pears: 6-foot-8, 316 pounds
Thomas Welch: 6-foot-7, 310 pounds
Mark Asper: 6-foot-7, 325 pounds
Cordy Glenn: 6-foot-6, 345 pounds
Kraig Urbik: 6-foot-5, 324 pounds
J.J. Unga: 6-foot-5, 320 pounds
Antoine McClain: 6-foot-5, 326 pounds
Bryant Browning: 6-foot-4, 325 pounds (practice squad)
Edawn Coughman: 6-foot-4, 313 pounds (practice squad)
Eric Wood: 6-foot-4, 310 pounds
Doug Legursky: 6-foot-1, 315 pounds