She had also used Twitter to announce the pregnancy in August.
According to a statement, the new baby weighs 8 pounds, 4.5 ounces and was born at 12:21 a.m. on Wednesday.
"Brian and I are overjoyed by the birth of our daughter," the congresswoman said. "Both the baby and I are healthy and doing well at the hospital."
McMorris Rodgers is now the first member of Congress to give birth twice while in office. Before her son Cole in April 2007, no child had been born to a sitting member in decades.
GOP Whip Eric Cantor (Va.) and Rep. Dan Burton (Ind.) tweeted their congratulations shortly following the birth announcement.