The 81-year-old, who left Live! in November 2011 after nearly 29 years, told USA Today that he will not be a permanent fixture on the series.
"I'm not a co-host, I'm just a guy showing up on her show once a month," he said. "I love the idea. It's fun to be back on TV when you don't have to do it every day."
Philbin, who described Ray as a "friend" and "a little dynamo", also revealed that he couldn't see any reason to turn down the opportunity.
"I'm not going to say, 'I don't want to do your show!'" he joked. "I think it's fun. I spent my whole life doing this... [But now] it's not the every-morning burden."
The show's executive producer Janet Annino praised the "chemistry" between Ray and Philbin, describing him as a "perfect fit".
Meanwhile, in a statement to TV Guide, Ray said: "I can't tell you how excited I am that Regis will be a recurring special guest on our show this season. Regis has been a close friend of the show and a close friend of mine for years, and it'll be a blast having him on the team."
Philbin is expected to make an appearance on The Rachael Ray Show on Monday.