Riggs, 31, was reportedly cleaning a pistol when it accidentally went off. He has been hospitalized for non life-threatening injures to his hand and thigh, the UFC said in a statement.
"We were made aware that newly-signed UFC competitor Joe Riggs was involved in an unfortunate accident last night," the UFC said. "While cleaning his permitted firearm, the gun discharged, injuring his hand and upper thigh. Riggs was transported to a nearby hospital in Arizona where is being treated by physicians. We wish Joe a speedy recovery."
Last week, Riggs (40-14) re-signed with the UFC and was scheduled to fight Paulo Thiago at a UFC Fight Night event Sept. 13 in Brasilia, Brazil.
UFC officials announced Sean Spencer has "verbally agreed" to replace Riggs in the Thiago fight. Spencer (11-3) fights out of Dallas and is 2-2 in the UFC.
Fighting out of Phoenix, Riggs has not fought in the UFC since 2006. He fought for multiple organizations in his time outside the UFC, including Strikeforce and Bellator MMA.
In November, Riggs won the Bellator MMA reality series "Fight Master," culminating in a unanimous decision against Mike Bronzoulis. He is currently on a six-fight win streak.