They're dealing with injuries at the position, with rookie Timmy Jernigan ruled out with a knee injury for Sunday's game against the Carolina Panthers and starting defensive end Chris Canty questionable with a knee injury.
Guy is a 6-foot-4, 318-pound lineman from Arizona State who has 38 tackles, one sack and two blocked field goals in three NFL seasons.
"We watched the tape and we liked the way he played," Ravens coach John Harbaugh said. "He fits our defense. They play a similar style defense out there, so we see where he fits in to what we do. He's a physical player, really a play-hard guy.
"We got a final confirmation from 'Double-J' when we texted him and he gave us his stamp of approval. So, [Johnson] said [Guy] has, 'Raven-like qualities.' So, that was all we needed to hear in the end."
Guy was cut by the Chargers after three games, in which he did not have a tackle.
Now, Guy gets a fresh start with the Ravens after arriving Thursday morning. He caught a red-eye flight from California and arrived in Baltimore at roughly 2:45 a.m. before reporting for his first day of work at team headquarters early that morning.
"It's amazing to get picked up by the Ravens," said Guy, who recorded 122 career tackles in college, including 23 tackles for losses and eight sacks. "They have great players here that I've looked up to since I was in college. I feel like I bring high-speed physicality to the table. I've had some playing time, but I've got to pick up this defense and learn it fast."