The bookmaker has penned a two-year deal with an option of a third to become the west Londoners' main team sponsor.
Marathonbet replaces FxPro and will have its logo displayed on the shirts worn by Fulham's first team and Under-21s.
"I am delighted that Marathonbet has joined us as our main team sponsor and we look forward to working with them in the future," Fulham chief executive officer Alistair Mackintosh said.
"As a dynamic and inventive brand, I believe that Marathonbet shares the club's objectives in terms of development and innovation."
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