Terms of the deal weren't available, but it is believed to be a one-year contract.
"Earlier in the offseason I wasn't ready or [didn't] have the mindset to come back and play football," McAlister said Wednesday, his first day with the club. "For it to take as long as it did, I'm happy it happened before I had no other opportunity."
A week ago Sunday, McAlister was cleared by doctors to play despite knee problems that have been nagging him for years. There were teams that were interested in signing him at the beginning of the season, but McAlister wanted to wait until his knee was completely healthy.
The 32-year-old McAlister is a three-time Pro Bowler. At 6-1, 206 pounds, McAlister is a physical corner who can help against the run, but for many years, he was considered one of the best corners in the game. He was the 10th pick in the 1999 draft by the Baltimore Ravens and played 10 seasons.
He had 26 interceptions and 339 tackles during his Ravens career.
The need for a cornerback was created by the groin injury that kept Jabari Greer out of Sunday's game against the Rams and the MCL knee sprain suffered by Tracy Porter Sunday that will sideline him indefinitely.
At 32, McAlister was bitter about being cut by Baltimore, the only team for which he'd played during a highlight-filled career that included three Pro Bowl invitations, a victory in the 2001 Super Bowl and a famous 107-yard touchdown return of a missed field goal.
"He's a veteran player that understands defense and is someone that we think can come in and help us," Saints coach Sean Payton said. "He worked out yesterday and did a good job."
McAlister injured the posterior cruciate ligament in his right knee in the middle of the 2007 season, but tried to play through it. He wound up playing in only eight games two seasons ago and six games last season before having season-ending surgery last November.
In February, while undergoing rehab, he was released by the Ravens. In June, he needed arthroscopic surgery to clean up debris in the same knee and he remained unsure about whether he wanted to attempt a comeback until the preseason rolled around.
"It was a bitter divorce in Baltimore, the way I was no longer a part of the club and for my first time in 10 years, didn't have any place to call home," McAlister said. "The offseason took on a new life for me. There was a transition period for me to figure out exactly if I'm going to come back and play football or am I not.
"The first preseason game started and I got butterflies like I was supposed to be coming out of a locker room somewhere," McAlister said. "I knew I was supposed to be somewhere else instead of on my couch."
Payton said the Saints are trying to get McAlister ready to play this Sunday, though it's unclear how much they'll need him.
McAlister has not played in more than a year. His last game was on Oct. 19th of last season. He remained on the active roster three more weeks before finally deciding to have surgery.
McAlister said he expects to catch on to the Saints' scheme quickly, given that defensive coordinator Gregg Williams, like former Baltimore coordinator Rex Ryan, were both disciples of Ryan's father, Buddy Ryan.
"They do the same thing," McAlister said. "There's just terminology difference."
McAlister said his body feels about as good as it has in several years, noting that he passed physicals before and after his workout with New Orleans.
"It's unfortunate, but fortunately I've been out almost a whole season with last year playing six games, the year before played eight games. And it's like my body is fresher," McAlister said. "Everything besides my knee is 100 percent fresh. I feel like I've still got a lot of bounce in my legs, and covering and doing my drills today I feel good."
John Clayton is a senior writer for ESPN.com. Information from The Associated Press was used in this report.