On Wednesday, the "10 Things I Hate About You" star announced the birth of her second child, Arlo, with husband Preston J. Cook.
"Welcome to the world, Baby Arlo! ✨ The newest addition to our family, reminding me how infinite love can be. 💖," Stiles captioned a photo of the newborn's feet.
"Scroll Forward to see how my four-year-old is taking it…." Stiles added, revealing her son, Strummer, covered their toilet with a red marker.
Stiles, 40, revealed in November that she was pregnant by cradling her growing baby bump on the red carpet of a screening for "The Humans" in New York City.
The actress wore a black turtleneck dress that hugged her midsection.
As for Stiles and Cook's love story, the pair started dating in 2015 before getting married in September 2017.
"Who doesn't love a shotgun wedding," the bride joked on Instagram when she said "I do" over Labor Day weekend. They welcomed Strummer one month later.