Riordan, a moderate and pragmatist who has sometimes aligned himself with Democrats, praised Lieu a "problem solver" in a statement released by the Lieu campaign Monday.
"He is smart, tenacious and effective," the statement continued, adding, "Ted has worked across the aisle with legislators and the executive branch for many years and made the tough choices that helped turn California's budget deficit into a budget surplus."
The endorsement could be helpful to Lieu as he tries to appeal to Republicans and unaligned voters in the 33rd Congressional District. Although Democrats hold a 44%-27% registration edge over Republicans, nearly one-fifth of district voters are not affiliated with any political party.
Riordan's backing also could help Lieu in the politically dominant Westside part of the district, where he is not as well known as in his South Bay home base.
One of Lieu's main competitors, former Los Angeles Controller and city Councilwoman Wendy Greuel, recently was endorsed by her successor, Controller Ron Galperin.
Although 21 people have filed to be on the June 3 primary ballot for Waxman's seat, only a handful have a significant public profile and are believed capable of raising the kind of money needed to campaign in the sprawling, largely coastal district.
Besides Lieu and Greuel, they include journalist and radio show co-host Matt Miller, a Democrat, and spiritual teacher and bestselling author Marianne Williamson, an independent.
The top two finishers in June, regardless of any party affiliation, will compete for the seat in the November general election.