The comedienne and the TV presenter are said to have been dating for the best part of this year, after Catherine's romance with Take That star Jason Orange ended.
They have been friends for years, but grew closer after Catherine appeared on Adrian's former ITV chat show That Sunday Night Show.
The couple, pictured together in London earlier in the summer, met up in the Santa Monica district of Los Angles between work commitments.
Forty-four-year-old Catherine is currently appearing in the US version of The Office while Adrian is hosting ITV's football coverage.
The pair were spotted canoodling in a park in the Californian sunshine, and onlookers said they seemed "besotted" with each other.
Adrian, 45, separated with his wife of 11 years, Radio Four presenter Jane Garvey, in 2008. The pair have two daughters – Evelyn, 12, and nine-year-old Sian.
Catherine also has a nine-year-old daughter Erin, from a long term relationship with stage manager Twig Clarke. They split up before summer 2011.