GlaxoSmithKline plc (GSK) (LSE: GSK, NYSE: GSK) is a British multinational pharmaceutical, biologics, vaccines, and consumer healthcare company headquartered in London, United Kingdom. It is the world's fourth-largest pharmaceutical company measured by 2009 prescription drug sales (after Pfizer, Novartis and Sanofi).

GSK has a portfolio of products for major disease areas including asthma, cancer, virus control, infections, mental health, diabetes, and digestive conditions. It also has a large consumer healthcare division which produces and markets oral healthcare and nutritional products and over-the-counter medicines including Sensodyne, Boost, Horlicks, and Gaviscon.

GSK has a primary listing on the London Stock Exchange and is a constituent of the FTSE 100 Index. As of December 2011, it had a market capitalisation of £73.8 billion, the fifth-largest of any company listed on the London Stock Exchange. It has a secondary listing on the New York Stock Exchange.

GSK was formed in 2000 by the merger of GlaxoWellcome plc (formed from the acquisition of Wellcome plc by Glaxo plc), and SmithKline Beecham plc (from the merger of Beecham plc, and SmithKline Beckman Corporation).