Parker's representatives confirmed the happy news to People after she was seen traveling with the child to the Emmy Awards show in Los Angeles this past weekend.
Representatives for the 43-year-old single mom, who was nominated for two Emmys, declined to reveal any more information about the adoption.
The "Weeds" star also has a 3-year-old son, Will, with actor Billy Crudup, her former longtime boyfriend.
Although the couple split up before Will was born, Parker told People last year she never worried about being a single mom.
"There are a lot of things I just would not attempt, but taking care of someone -- I know how to do," she said. "I know how to do that and I enjoy doing that."