Bellass, who has been with Mother for eight years, will join Splendid in March. He will share equity with Alec Samways, the founder of Splendid, and the agency's PR director, Kathryn Pringle.
Splendid has worked with clients including Diageo, Unilever, Yahoo! and Xbox.
Bellass said he was motivated by a growing trend towards brands with which consumers could interact. He cited Nike's "Run London" campaign as an example.
He added: "The best area for growth for me personally, and the marketing industry as a whole, is the creation of communication ideas that engage people on a physical level."
Bellass starts his new role in March. The shares in Mother Bellass holds will revert back to the agency, which is standard company policy.
Splendid was founded by Samways in 2003, and has held high-profile events including a purification installation for Smirnoff, which turned water from the Thames into drinking water in 2005.