The actress, who plays Dr Izzie Stevens in the ABC medical drama, confirmed the adoption during an appearance on The Ellen DeGeneres Show.
"They sent us a picture. They told us that it was a little girl and who she was and the information about six months ago," she explained.
"It was meant to take a long time. This sort of thing can take years. She has special needs. She is a special needs baby and because of that it all moved so much faster. They wanted to get her to us as quickly as possible."
Heigl, who has yet to meet her new daughter, went on to say that the couple had already named her Naleigh but that the name could change if they decided that it did not suit the new arrival.
She added: "We started talking about adoption even before we were engaged because it's really important to me. It's been a big part of my life and my family.
"My sister is Korean and my parents adopted her back in the '70s and so I just always knew that this is something I wanted to do. I made that really clear because not everybody is down with that."
Heigl's full interview will air on Friday.