"We are incredibly grateful that our family will be growing by two," the pair wrote, and "we thank you for your well wishes".The post, signed "The Carters", has a photo of Beyonce with a baby bump, wearing lingerie and a veil.
The couple already have a daughter, Blue Ivy, who has just turned five.
Beyonce has been nominated in nine categories for the 2017 Grammy Awards, extending her lead as the most-nominated woman in Grammys history.
The star, 35, was due to headline the Coachella music festival in southern California in April.
The announcement gave no indication of the babies' due date.