Costner, 55, and his wife Christine Baumgartner, 36, welcomed their third child, a baby girl named Grace Avery Costner, on Wednesday, June 2.
Grace Avery arrived at 5:46 p.m. in California. She weighed 7 lbs., 11 oz., and was 21 inches at birth. The couple's rep tells People magazine that both mother and daughter are doing well.
The "Dancing with Wolves" star married Baumgartner, a German model, back in 2004. They have two other sons together - Hayes Loan, 16 months, and Cayden Wyatt, 3. Costner has four other children from a previous marriage. His eldest child is 25 years old.
Costner recently made headlines with his company, Ocean Therapy Solutions. BP officials agreed to test his company's oil-cleaning device, which could separate oil from water, as part of an effort to clean up the massive oil spill in the Gulf.