The UK's biggest pharmaceuticals group bought Maxinutrition Group Holdings, Europe's top-selling sports nutrition company, from Darwin Private Equity for 162 million.
The muscle-building powders will expand Glaxo's portfolio of brands that already includes Ribena, Sensodyne toothpaste and Lucozade.
Maxinutrition - which also owns Maxifuel dietary supplements and Maxitone muscle toning supplements for women - has increased its sales by 21 per cent over the past three years to 36m in the year to April.
John Clarke, Glaxo's president of consumer healthcare, said, "This deal will give (Glaxo] a strong presence in the fast developing protein-based sports nutrition market, appealing across a broad spectrum of consumers from elite athletes to sports participants."
The brands are currently sold in Europe but Glaxo will market them around the world.
Last month, Glaxo announced it would create about 1,000 jobs and invest 500m in manufacturing.
Glaxo's production operations in the UK include sites at Montrose and Irvine.