The 9x21mm Russian round is a modern pistol cartridge of Russian origin. Its development started in the 1980's in the Soviet Union under the Grach program. Three rounds resulted from this program. These are the very hot loaded 7N21 and 7N31 (aka PBP) 9x19mm rounds and a 9x21mm round that is also known as the SP-10. The Russian military shifted its attention in the mid 1990's towards the 9x19mm versions, but several government institutions adopted the 9x21mm round and it is slightly becomming a more common round in Russia. The Russian 9x21mm round should not be confused with the less powerful 9x21mm IMI round.

The 9x21mm round is best described as a 9x19mm round with much hotter loading. It uses a straight rimless case with extractor groove. The case is longer than in the original 9x19mm round in order to hold more propellant. The hot load results in a higher kinetic energy, longer range and better armor piercing performance. The original SP-10 bullet developed in the Grach program is the most special in the 9x21mm lineup. It features a steel penetrator protruding from a metal jacket. Other 9x21mm rounds are more conventional.

The original weapon system developed for the 9x21mm round was the high capacity Gyurza pistol. The Gyurza is also known as the P-9, SR-1 Vektor and SPS. To complement the Gyurza the SR-2 Veresk sub machine gun was developed several years later. These weapon systems are the only ones to use the 9x21mm round. Foreign weapons chambered for the 9x21mm IMI round cannot fire this Russian round due to differences in dimensions and much higher chamber pressure.

Compared to the standard 9x19mm round the 9x21mm round has a much better lethality, longer range and improved armor piercing capabilities. When compared to the very hot loaded Russian 7N21 and 7N31 9x19mm rounds the performance gap is smaller. The SP-10 armor piercing round is capable of penetrating 30 layers of Kevlar plus two 1.6 mm titanium plates at 50 meters. The maximum effective range of this round is about 200 meters.

The 9x21mm round is a rather uncommon round that is only in use with units that use the Gyurza pistol or SR-2 sub machine gun.