Our real estate sources tell us Lori and Mossimo sold their massively expensive home to Tinder co-founder Justin Mateen, and he got quite a deal at $18,750,000.

TMZ broke the story ... Lori and Mossimo quietly put the property back on the market for $28,650,000 after previously listing it in 2017 for $35 million.

We'll spare you the math ... the Tinder honcho scooped up Lori and Mossimo's pad for $9.9 million less than their most recent asking price.

Justin didn't just get a reduced price, the house is pretty great -- 12,000-square-feet, 6 bedrooms, 9 baths and all the bells and whistles -- and, of course, he's now in one of L.A.'s most ritzy 'hoods.

Arvin Haddad, who is with The Agency and stars in CNBC's "Listing Impossible," held the listing while Josh Flagg of Bravo's "Million Dollar Listing Los Angeles" and Rodeo Realty repped the buyer.

Now, Lori and Mossimo will turn to their next home challenge. Remember, they've made a habit of selling their family home every 3 to 5 years ... and he's super talented at remodeling houses.