The stake, the size of which was not known, is the latest investment by one of France's richest men. Niel set up low-cost telecoms operator Iliad in France and this week was said to be eyeing a move into the U.K. market.
The specific goal behind the move was unclear, but a spokesman for AMR said that Niel wanted to diversify his portfolio, which also includes a stake in Telecom Italia.
Guinea is a major producer of bauxite, an aluminium ore, but growth has been hamstrung by a slump in metals prices and a two-year Ebola epidemic in the West African nation that killed more than 2,500 people and drove away investors. Guinea was declared free of the virus on Dec. 29.
Guinea President Alpha Conde last month named mining executive Mamady Youla as prime minister as part of a strategy to revive the country's ailing economy, the government said.