Serena Williams and Alexis Ohanian have a lot to celebrate ... the couple just welcomed their second child together!!
The tennis superstar revealed the major news with a TikTok on Tuesday ... showing her get up from a table and leave the frame before returning with the little bundle of joy.
Alexis and Olympia look happy as can be -- with big sis adorably giving baby Adira River a kiss on the forehead.
The Reddit co-founder also posted about the newcomer .