The 155-pound bout is the first official addition to the card, which takes place July 27 at KeyArena in Seattle. UFC officials broke the news via Twitter on Wednesday.
Guillard (30-12-2) is badly in need of a win, having lost four of his past five fights. The 30-year-old veteran worked his way toward title contention in mid-2011 with a five-fight win streak, but he has plummeted since.
Guillard tweeted recently he would hold his training camp at Grudge Training Center in Denver, Colo. He made a recent split with the Blackzilians camp in South Florida but was reportedly denied access to his former team at Greg Jackson's MMA in Albuquerque, N.M.
Danzig (21-10-1) comes off a split decision loss to Takanori Gomi at a UFC on Fuel event in November. Winner of the sixth season of The Ultimate Fighter reality series, Danzig holds a 5-6 overall record in the Octagon.