The S-400 Triumf (Russian: C-400 «Триумф») is a new generation anti-aircraft weapon system developed by Russia's Almaz Central Design Bureau as an upgrade of the S-300 family. It is currently in limited service with the Russian Armed Forces.

The S-400's NATO reporting name is SA-21 Growler, and the system was previously known as S-300PMU-3. Its range is at least twice that of the previous S-300PMU system.

The S-400 uses 3 different missiles to cover its entire performance envelope. These are the extremely long range 40N6, long range 48N6 and medium range 9M96 missile. Each one has different capabilities.

The development of the S-400 system began in the late 1990s. The system was formally announced by the Russian Air Force in January 1999. On 12 February 1999, the first tests were performed at Kapustin Yar in Astrakhan which were reported successful. As a result of this, the S-400 was scheduled for deployment in the Russian army in 2001.

In 2003 it first became apparent that the system was not ready for deployment, but in August two high ranking military officials expressed concerns that the S-400 was being tested using 'obsolete' interceptors from the S-300P and concluded that it was not ready for production.