The Legally Blonde star, who gave birth to her third child in September (12), will be honoured by America's March of Dimes organisation at a gala luncheon in California later this week (07Dec12).
Witherspoon will receive the group's Grace Kelly Award for being a good role model.
The prize will be presented to the actress at the March of Dimes Celebration of Babies event at the Beverly Hills Hotel on Friday (07Dec12).
The 36 year old became a mother again in September (12) when she gave birth to a son called Tennessee, her first child with second husband Jim Toth. She has two other kids, Ava, 13, and Deacon, eight, from her previous marriage to Ryan Phillippe.
March of Dimes was founded by U.S. President Franklin D. Roosevelt in 1938 to combat polio and now works to help women through pregnancy and new motherhood.