RNA - The mosque is located on Rue de Tanger in the city's 19th administrative district, a religiously and culturally diverse part of the capital, Anadolu news agency reported.
The gunman, wearing a helmet, fled the scene on a scooter, according to local sources.
The victim, a 32-year-old man, has been rushed to the hospital with multiple gunshot wounds.
Around 15 people were inside the mosque at the time the gunman entered. No other injuries have been reported.