The 30-year-old has signed a two-year deal with Ian Holloway's men after leaving West Brom earlier in the summer.
Thomas' departure from The Hawthorns ended his four-year stay with the Baggies and he has immediately joined his new team-mates at their pre-season camp in Portugal.
Thomas, who started his career with Arsenal but has played for the likes of Charlton, QPR, Portsmouth and Leeds, said Palace's new status plus the lure of working under Holloway had persuaded him to join the Selhurst Park club.
"It was a case of me being on a free and weighing up my options. I wanted to stay at the top level, which is obviously the Premier League," he told his new club's official website.
"I heard of Crystal Palace's interest from January actually, and once they got promoted Ollie gave me a call when I was away on holiday, we had a good talk for about 45 minutes and after that conversation there wasn't much doubt in my mind where I wanted to go.
"I saw them throughout the season and watched the play-offs. Like I said, Ollie's the main factor, but obviously there's a lot of talent here and him and the chairman are planning on bringing in more. It shows what their goals are and I'm definitely looking forward to it.
"I hope I can bring a lot to the team. I've got experience of the Premier League and the gaffer's going to give me the freedom to play how I want to play - my attacking game - which I haven't been able to do for the last couple of seasons. That's what I'm looking forward to."