"The most perfect Mother's day...I wish all you Mama's the happiest of days today!," she wrote.
The Stapletons haven't yet shared what day the baby was born or if baby No. 5 is a boy or a girl. The couple, who married in 2007, haven't released the names of any of their children to the public.
Stapleton announced that he and his wife were expecting during a concert at Madison Square Garden in New York last November.
"I've got an announcement to make. Some of you may know that we have four children, and we just had twins about seven months ago," Stapleton said at the show. "Now there's four for us, but we're gonna make it five."
The couple had twin boys in April of 2018. The twins' arrival was announced by Reba McEntire during the 2018 ACM Awards.
Stapleton recently teamed up with Pink on the ballad "Love Me Anyway," featured on the pop singer's new album Hurts 2B Human.