The Reddit co-founder, borrowing a tactic at the centre of Oscar-nominated movie Three Billboards Outside Ebbing, Missouri, delighted Ms Williams after the giant adverts appeared on the motorway heading to the desert city of Indian Wells.
The first three billboards, which also feature images of the couple's baby daughter Alexis Jr, spell out the message, "Greatest Momma Of All Time". Ain't that adorable?
A fourth billboard reads: "Serena Williams - G.M.O.A.T - Alexis Jr + Sr."
"These just went up alongside I-10 into Palm Springs," Mr Ohanian wrote on Instagram. "I wanted to welcome her back to tennis. Designed them myself with some help from Jr," he added.
See the billboards for yourself below:
Ms Williams was bowled over by the grand gesture. "Literally am crying," she wrote in a comment on Mr Ohanian's Instagram post. "This is so sweet. I love you."
"This is amazing," commented tennis star Caroline Wozniacki.
Ms Williams, 36, has not played a full WTA tournament since missing most of 2017 due to the birth of her daughter.
The 23-time Grand Slam champion played in a Fed Cup game for the United States earlier this month. Mr Ohanian was on "dad duty" then: as the tennis star played her match on court, her five-month-old daughter cheered her on from the stands, in Mr Ohanian's arms.
Ms Williams will return at next week's BNP Paribas Open at Indian Wells.
(With inputs from AFP)
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