The twins, named Darby Galen and Sullivan Patrick, were born in Los Angeles. They join 4-year-old sister Talula in the Dempsey household.
Dempsey, 41, who plays Derek "Dr. McDreamy" Shepherd on Grey's Anatomy and recently starred in the movie Freedom Writers, announced the pregnancy in September on Live with Regis and Kelly.
In January, he posed for Life with his head resting on Jillian's pregnant belly.
Asked how fatherhood has changed him, he told the magazine, "Now, making money is about providing for my children. And being a father makes you look at yourself. You look at your marriage and go, 'How do I improve this? How do I keep growing and create a stable environment for my children?' "
Dempsey and his wife met in 1994 when he came into her salon for a haircut. (She now has her own high-end cosmetics line.) They married in 1999 at the Dempsey family farmhouse in Maine.