TJX Cos. Inc., the Framingham-based owner of T.J. Maxx and Marshalls stores, said it bought discount online retailer Sierra Trading Post for about $200 million.

The privately-held Sierra Trading Post, which launched in 1999 and is headquartered in Cheyenne, Wyoming, generates annual revenue of more than $200 million. The company specializes in apparel, footwear, and home furnishings.