S99

S99 may refer to:
 * Avia S-99, a Czechoslovak fighter aircraft
 * SAS Assegaai (S99), formerly SAS Joanna van der Merwe, a submarine of the South African Navy
 * Sintura S99, a racing car
 * SOVIET SUBMARINE S-99, a submarine of the Soviet Navy
 * S99, a refugee in Nauru
 * S99, a line of the Lucerne S-Bahn