GSK plc, formerly GlaxoSmithKline plc, is a British multinational pharmaceutical and biotechnology company with global headquarters in London, England. Established in 2000 by a merger of Glaxo Wellcome and SmithKline Beecham, GSK is the tenth largest pharmaceutical company and #294 on the 2022 Fortune Global 500, ranked behind other pharmaceutical companies China Resources, Sinopharm, Johnson & Johnson, Pfizer, Roche, AbbVie, Novartis, Bayer, and Merck.

The company has a primary listing on the London Stock Exchange and is a constituent of the FTSE 100 Index. As of August 2022, it had a market capitalisation of £70 billion, the eighth 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.

In 2012, under prosecution by the United States Department of Justice (DoJ) based on combined investigations of the Department of Health and Human Services (HHS-OIG), FDA and FBI, primarily concerning sales and marketing of the drugs Avandia, Paxil and Wellbutrin, GSK pleaded guilty to promotion of drugs for unapproved uses, failure to report safety data and kickbacks to physicians in the United States and agreed to pay a US$3 billion (£1.9bn) settlement. It was the largest health-care fraud case to date in the US and the largest settlement by a drug company.