The 24-year-old, whose contract with current club Wolfsburg expires in the summer, has committed to 'The Eagles' till 2027.
Usually deployed as a winger, Marmoush has scored five goals in 31 league games this season.
Born in Cairo with Egyptian and Canadian citizenship, Marmoush has played 17 times for Egypt, scoring four goals.
Frankfurt sporting director Markus Kroesche said in a statement: "Omar has already proven his qualities in the Bundesliga and at the same time still has great potential for development and improvement."
Presently eighth in the league, Frankfurt agreed to part ways with their Europa League-winning coach Oliver Glasner at season's end last Tuesday.
Eliminated from European competition, Frankfurt will take on holders RB Leipzig in the German Cup final in early June.