The 4K51 Rubezh system is a late Cold War era coastal defense system of Soviet origin. It is best known by its NATO designation SS-C-3 Styx. The 4K51 is a much less complex design than the previous 4K44 Redut (SS-C-1 Sepal) and S-2 Sopka (SS-C-2 Samlet).

For a coastal defense system the 4K51 uses surprisingly little components. All subsystems are fitted onto one vehicle. This vehicle is based on the large MAZ-543 8x8 truck chassis. The driver's cab and engine are mounted at the front. The command module and radar are in the center of the vehicle. The large twin round launcher is fitted at the rear.

The 4K51 fires the large P-15 Termit missile, known in the West as the SS-N-2 Styx. The P-15M versions with mid course updates, the P-20 version with active radar guidance and P-21 version with infra-red guidance can be launched. The large twin round launcher is rotated either to the left or right, in the direction of the target. Normally each launch unit is accompanied by two 6x6 trucks outfitted with a crane and a spare missile.

The command module houses the remaining four crew members needed to operate the weapon. On top of the command module the MR-331 Rangout (Square Tie) or 3Ts-25 Garpun surface search radar is fitted, which is raised when in use. The 4K51 can operate autonomously using its own radar to detect and track targets. However, it is mostly used in conjunction with other means of detection, such as land based radars or aircraft.

The 4K51 is based on the massive MAZ-543 high mobility truck. Although it is a heavy and large system the large 8x8 truck provides good all terrain performance. In the mid-1980's the system was upgraded by using the improved MAZ-543M chassis.

The foremost user of the 4K51 system has been the USSR. Like the ship based version of the Termit missile the 4K51 was widely exported. It was mainly adopted by nations in Eastern Europe and the Middle East.