Wittich leaves role as F1 race director with immediate effect
The FIA said Wittich, a German who has carried out the role since 2022 after Australian Michael Masi was removed from the position, had left "to pursue new opportunities".
"Niels has fulfilled his numerous responsibilities as race director with professionalism and dedication," it added in a statement.
"We thank him for his commitment and we wish him the best for the future."
Marques, a Portuguese, has previously served as an international steward and race director for the Formula Two and Formula Three championships.
The Formula One championship has three rounds remaining with both titles still to be decided. Red Bull's Max Verstappen could win his fourth successive drivers' crown in Las Vegas.