The stars played respectively Ariel, Prince Eric, Sebastian, Chef Louis and King Triton.
Entertainment Weekly said Saturday night's show at The Hollywood Bowl featured a live orchestra and songs from the classic, animated 1989 Disney movie.
"Thank you so much to all the fans who came out to see The Little Mermaid in Concert last night," Wilson tweeted after the show, along with photos of her dressed in her Ursula the Witch costume.
"This is my dream. And it just happened. Omg. #gratefulmermaid," Bareilles said in her own Twitter post, which captioned a group photo of her with Lewis, Wilson, Criss, Burgess, Stamos and others.