The Cleveland Cavaliers player and his ex, Jordan Craig, welcomed a son last week, a source close to Craig confirms to ET.
"Both mom and the baby are happy and healthy!" the source says.
Neither parent has announced their baby's birth yet, but on Nov. 30, Craig shared a photo on Instagram, writing, "Going to miss my bump ... but the best is yet to come!"
On Sunday, Kardashian shared her own picture on Twitter, showing herself in an embrace with Thompson while the pair sported matching camouflage jackets.
"Him," Kardashian wrote along with a heart emoji.
The 25-year-old NBA champ and 32-year-old reality star went public about their relationship in September.
In the January issue of Health magazine, Kardashian opened up about what attracted her to Thompson.
"My boyfriend's a cutie, so I do like big men," she admitted. "I love athletes. Someone who has the same interests as me, who likes to work out, like that's their hobby or their job, that's fine."
"Tristan is who I'm into," she stated explicitly. "He's a little cutie."