The NFL did not disclose financial terms of the deal.
Kroenke, whose fortune Forbes magazine has estimated at $2.7 billion, also owns the NBA's Denver Nuggets, the NHL's Colorado Avalanche, Major League Soccer's Colorado Rapids and National Lacrosse League team Colorado Mammoth.
"Stan has been a familiar and respected figure in the National Football League for more than 15 years," said NFL Commissioner Roger Goodell. "He is a proven businessman and has experienced success in all of his sports franchises, as well as serving as a responsible community leader."
Prior to becoming the majority owner of the Rams, Kroenke was the minority owner of the team, which was controlled by Chip Rosenbloom.
The American owns 29.9 percent of London club Arsenal, who finished third in the Premier League last season.
A real-estate and sports investor, Kroenke has a regional sports TV network, a sports arena and soccer park, and a real estate portfolio that includes shopping centers, office and apartment buildings throughout North America.
The Rams franchise, which began in Cleveland, was in Los Angeles from 1946 to 1994 and took up home in St. Louis in 1995 in a move that brought the NFL back to the city after an eight-year absence.
Prior to the Rams, the St. Louis Cardinals were the city's NFL team from 1960 to 1987 before they moved to Phoenix and later became the Arizona Cardinals.
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