TJX Cos. said Friday it bought discount Internet retailer Sierra Trading Post for $200 million, a deal that is expected to help the Framingham company launch e-commerce sites for its T.J. Maxx, Marshalls, and HomeGoods chains.

Privately held Sierra Trading Post, based in Cheyenne, Wyo., was founded in 1999. It has about 700 employees and annual revenue of roughly $200 million through sales of apparel, footwear, and home furnishings.