GlaxoSmithKline plc (GSK) is a British multinational pharmaceutical company headquartered in London, England. Established in 2000 by a merger of Glaxo Wellcome and SmithKline Beecham, GSK was the world's sixth largest pharmaceutical company according to Forbes as of 2019, after Pfizer, Novartis, Roche, Sanofi, and Merck & Co. GSK is the tenth largest pharmaceutical company and #296 on the 2019 Fortune 500, ranked behind other pharmaceutical companies including China Resources, Johnson & Johnson, Roche, Sinopharm, Pfizer, Novartis, Bayer, Merck, and Sanofi.

The company has a primary listing on the London Stock Exchange and is a constituent of the FTSE 100 Index. As of August 2016, it had a market capitalisation of £81 billion (about US$107 billion), the fourth largest on the London Stock Exchange. It has a secondary listing on the New York Stock Exchange.

The company developed the first malaria vaccine, RTS,S, which it said in 2014 it would make available for five percent above cost. Legacy products developed at GSK include several listed in the World Health Organization's List of Essential Medicines, such as amoxicillin, mercaptopurine, pyrimethamine, and zidovudine.