The singer and Fashion Star mentor posted a Christmas Day photograph of her daughter Maxwell Drew Johnson in front of a sand drawing which reads 'Big Sis'.
"Merry Christmas from my family to yours," Simpson wrote on her official Twitter page.
It was reported in November that Simpson and fiancé Eric Johnson were expecting a second baby in 2013.
However, representatives for the couple declined to comment at the time.
The couple welcomed Maxwell in May of this year. Simpson back then announced she was expecting via her official website, posting a picture of herself dressed as a mummy for a Halloween party.
Simpson was recently quoted as saying that becoming a mother has "drastically changed" her life.