The deal, worth the equivalent of 192 million euros or $255 million, is subject to regulatory approval, the pair said in a joint statement.
"GlaxoSmithKline and Maxinutrition Group Holdings Limited today announced they have entered into an agreement for GSK to acquire Maxinutrition, a UK company that manufactures protein-enhanced functional nutrition products, from Darwin Private Equity," they said in the statement.
"Subject to approval from the Office of Fair Trading, GSK will acquire 100 per cent of the shares of Maxinutrition for a cash consideration of approximately 162 million pounds including the repayment of outstanding debt."
Maxinutrition produces protein-enhanced products that are aimed at consumers who want to increase their muscle tone, power and strength. Its top-selling brands are Maximuscle, Maxifuel and Maxitone.
The acquisition will bolster GSK's portfolio of consumer healthcare brands that already include Lucozade sports drinks and Sensodyne toothpaste.
"This deal will give GSK a strong presence in the fast developing protein-based sports nutrition market," said GSK's consumer healthcare president John Clarke in the statement.
The Maxinutrition brands would appeal "across a broad spectrum of consumers, from elite athletes to sports participants and those seeking additional nutritional supplementation".
He added: "The acquisition is a demonstration of GSK's strategy to expand our consumer healthcare business through appropriate bolt-on acquisitions which meet our strict financial criteria."
Maxinutrition, Europe biggest sports nutrition company by market share, enjoyed sales of about 36 million pounds in its financial year to the end of April.