Left-hander Josh Newman was optioned to Triple-A Omaha to make room for Wells.
Wells, 31, had spent this season in the Rockies organization. He was designated for assignment last week.
"We signed him to pitch out of the bullpen for us," Royals manager Trey Hillman said.
Hillman wasted no time in getting Wells into a game. He pitched 1 1/3 innings on Tuesday night, giving up two runs, three hits and two walks in a 9-4 loss to the Indians.
Although Wells started on Opening Day for the Rockies, he pitched most of the season out of the bullpen and had a 1-2 record with a 5.27 ERA in 15 games, including two starts.
Wells missed 2 1/2 months after undergoing surgery for a blood clot in his right hand, rejoining the Rockies in late July.
"I was having circulation problems in my hand to where I was losing sensation, so once they figured out what it was, I was on the DL for basically 75 days," Wells said. "But part of that was, I was on blood thinners that had to run their course before the doctor would allow me to get back into competition."
To remove the clot, doctors cut into his right forearm and used a balloon procedure through his artery.
"Reports are that he's healthy," Hillman said. "I've spoken to some people about his condition this season and he's somebody we looked into as soon as we were a little bit light [in the rotation.] As far as I know, he's healthy and ready to go."
The bullpen has been hampered by a foot injury to left-hander Ron Mahay.
Although the Royals have not yet announced a starting pitcher for Saturday's game against the Tigers, Hillman said he's not projecting Wells for that assignment.
"We'll utilize him from the sixth inning on," the manager said. "In the majority of his career, he's been a starter, but he was signed with the understanding that he'd pitch out of the bullpen, until there's further evaluation and see where we're at."
The look at Wells won't cost the Royals a lot, because the Rockies are paying Wells' guaranteed $3.1 million salary for this season. The Royals will be liable for about $90,000.
In 10 years with the White Sox, Pirates, Rangers, Cardinals and Rockies, Wells has a 65-93 record with a 4.64 ERA in 246 games including 205 starts. His best year came in 2003 for the Pirates, when he was 10-9 with a 3.28 ERA in 31 starts.
Newman appeared in three games for the Royals, with no record and a 4.50 ERA. He was called up from Omaha on Aug. 9 when lefty Horacio Ramirez was traded to the White Sox.
He gave up six runs (three earned) in six innings. In his last outing on Sunday against the Yankees, Newman relieved starter Brian Bannister in the second inning and turned in four innings, allowing just one run.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.