The 21-year-old had been linked with a move to Arsenal this summer, with Chelsea and Manchester United also believed to have identified him as a target.
The Slovakian joined RB Leipzig from RB Salzburg in 2023, signing a five-year contract that contained a £55m release clause.
Sesko, who netted 14 goals in 31 games during his debut campaign in Germany, has committed his future to the club until 2029.
“I had a good first year at RB Leipzig and am incredibly happy to be here,” he said.
“Team, club, city, fans – the overall package is simply outstanding for me. The early contract extension was therefore the logical next step.”
