Nathan Peterman is the only other quarterback on the Raiders' roster.
Glennon, 29, was released by the Arizona Cardinals on March 8 after just one season with the team. He appeared in two games with Arizona, completing 15 of 21 passes for 174 yards and a touchdown.
The Chicago Bears signed Glennon in 2017 to be their starter, but later traded up to the No. 2 overall pick in the draft to select quarterback Mitchell Trubisky. Glennon opened the 2017 season as the starter but struggled in four games before being benched in favor of Trubisky. Chicago released Glennon after the season.
He began his career with the Tampa Bay Buccaneers after being selected in the third round (73rd overall) of the 2013 draft.
In 27 games (22 starts), he has completed 60.9 percent of his pass attempts for 5,107 yards, 35 touchdowns and 20 interceptions.
Riley also set career-highs with 73 tackles and five passes defensed. He played his first three NFL seasons with the Tennessee Titans.