The Oscar-winning actress, who gave birth to her third child last month, might collaborate on a track with Buble, reported Us magazine.
Witherspoon has displayed a skill in singing when starring in "Walk the Line" in which she won an Oscar as country singer June Carter.
She did training and voice lessons to prepare for the role, calling the experience "the most challenging, horrifying experience of [her] life."
"I'd never sung professionally," she said in an interview. However, her performance was eventually well-received by movie critics including Roger Ebert.
The 36-year-old actress has been seen out and about after giving birth to Tennessee James on September 27. She was shopping for new clothes in Beverly Hills on Wednesday, October 24 with a female friend. She also took son Deacon to a soccer game last weekend at a Los Angeles soccer field near her Brentwood, California home.