Rihanna and A$AP Rocky are expanding their family ... because pregnant RiRi just debuted her new baby bump on the way to the Met Gala in NYC!

Our sources confirm ... Rihanna and Rocky are expecting their third child -- but you could tell that just by looking at her in Manhattan Monday!

RiRi gave birth to their first son on May 13, 2022 ... naming him RZA Athelston Mayers, a nod to the famous Wu-Tang Clan member RZA. The couple waited almost a year to publicly reveal his name.

Rihanna announced her second pregnancy in a BIG way ... showing off her baby bump in February 2023 during her Super Bowl LVII halftime show. She rubbed her belly during the performance and the millions tuning in around the globe wondered if she was pregnant, which she confirmed after the game.

RiRi and A$AP's second child was born August 3, 2023 ... and they named him Riot Rose Mayers. Rihanna later revealed she and Rocky decided on Riot for a name after a suggestion from Pharrell Williams, who recorded a song with Rocky called "Riot."

We're not sure yet how far along she is, but we've reached out to RiRi's reps for comment ... so far, no word back.