Rumors saying Simpson is expecting again have circulated for weeks, but she remained silent until Tuesday, UsMagazine.com reported.
She tweeted a Christmas Day photo of her 7-month-old daughter Maxwell, wearing red and white pajamas and sitting on a beach with "Big Sis" written in the sand.
"Merry Christmas from my family to yours!" Simpson tweeted.
She did not disclose the new baby's gender or anticipated due date.
Simpson, 32, is engaged to former San Francisco 49ers and New Orleans Saints tight end Eric Johnson, 33.
Simpson's hit records include "I Wanna Love You Forever," "Where You Are" and "Heart of Innocence." Her film credits include "Blonde Ambition" and "The Dukes of Hazzard."