Both had wanted to host the 2024 event.
The IOC welcomed Los Angeles' move, pledging to contribute $1.8bn (£1.4bn) to the organising committee.
A source close to the negotiations told BBC Sport that for LA to have accepted the 2028 Games there would have had to have been significant financial benefits that outweighed the benefit of hosting in 2024.
The funds will include money to pay for the extended planning period and to support youth sports programmes leading up to the Games.
Los Angeles bid committee chairman Casey Wasserman earlier said "both cities now stand together ready to serve the Olympic and Paralympic movements".
Meanwhile, Los Angeles Mayor Eric Garcetti said "we're bringing the games back" in 2028, adding that "this deal was too good to pass up".
Los Angeles has staged the Olympics twice - in 1932 and 1984.
Paris Mayor Anne Hidalgo said: "Paris and Los Angeles are two amazing global cities that are united in their support of the Olympic cause and we stand together now to help the Games thrive in 2024 and 2028.
"As today's announcement shows, dialogue between the IOC and the two cities is progressing well. We are confident that a 'win-win-win' agreement can be finalised ahead of the IOC session in Lima."
The IOC wanted LA and Paris to reach an agreement before the session in September and had they not, a vote to select the 2024 host would have been held.
IOC president Thomas Bach welcomed LA's decision, describing the bid as "strong and enthusiastic".
He added: "We are very confident that we can reach a tripartite agreement under the leadership of the IOC with LA and Paris in August, creating a win-win-win situation for all three partners.
"This agreement will be put forward to the IOC session in Lima in September for ratification."
In June, Wasserman appeared to concede to Paris when he said: "We have never been only about 2024."
However, a spokesman later told BBC Sport: "LA is the ideal host city for 2024 and is not conceding anything in the race for the 2024 Games."
Paris has said it has to go first as the site it has earmarked for the Olympic Village will not be available for redevelopment after 2024.
Los Angeles had argued that it could host a low-cost Olympics given that it already has all the necessary infrastructure in place, while Paris would need to build several expensive structures.
The two cities were the only ones left in the race for 2024 after Budapest, Hamburg and Rome withdrew.
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