CoStar Group, Inc. is an American provider of information, analytics, and marketing services to the commercial property industry in North America and Europe.

[13] From 2017 through 2020, the company acquired the online marketplaces Westside Rentals,[14] ForRent.com (previously owned by Dominion Enterprises),[15] Cozy Services,[16] Off Campus Partners,[17] and Ten-X;[18][19] in addition to the hotel research and analytics firm STR,[3] the residential mobile application provider Homesnap.

[22][23] Overseas, its European division acquired the German real-estate business data company Thomas Daily and the Spanish online marketplace and information provider Belbex in 2016.

[33] CoStar has been criticized for anticompetitive and monopolistic business practices, often using aggressive litigation and "public-relations warfare" to "push [competitors] to the brink of collapse or weaken them enough to make them soft targets for an acquisition".

[34] In February 2024, a proposed consumer class action lawsuit was filed against CoStar in multiple states, accusing the company of a price-fixing conspiracy in which it conspired with a group of luxury hotel chains—including Hilton, Hyatt, Marriott, InterContinental, Loews, and Accor—to keep room rental prices artificially high by sharing competitively sensitive information through the company's STR reports.