The American Idol alum shared a picture on her profile with baby Rennie David Foster in what she captioned as "Mommy & Me."
In the photo, the 37-year-old mom appeared to be drinking a Blue Moon beer while baby Rennie played in a yellow, green and pink floaty in a small pool. The blow-up pool was set in the middle of a room, and the mom rocked a black sports bra in the water.
Katharine has been playful with her son on Instagram before. She shared a video holding him up to a television screen of her singing in an April post and said, "The ONLY screen time my baby is allowed to have."
Her son with David Foster, who she married in 2019, was born back in February. She explained the meaning behind Rennie's name on Today with Hoda and Jenna.
"OK, well, his name, we haven't said anything, my husband hopefully won't kill me for saying it, but his name is Rennie David Foster," Katharine revealed to Hoda Kotb and Jenna Bush Hager. "We picked Rennie because I had actually been in labor for a while, we didn't have a name picked out."
"It was his great-grandfather's name, his great-uncle's name," the star added, "so it has a long history in his family. My husband said, 'Hi, Ren Foster,' and so we said, 'That's a good name. It's a strong name.'"
David has five other children: Erin Foster, Amy Foster, Sara Foster, Allison Jones Foster and Jordan Foster. Rennie is Katharine's first child.