The T-90S is in service with the Russian Army and has been selected by the Indian Army, with a requirement for 250 vehicles. Three T-90S are undergoing trials in the Indian desert.

The T-90S armament includes one 125 mm 2A46M smoothbore gun, stabilised in two axes and fitted with a thermal sleeve. The gun tube can be replaced without dismantling inside the turret. The gun can fire a variety of ammunition including APDS (Armour Piercing Discarding Sabot), HEAT (High Explosive Anti-Tank), HE-FRAG (High Explosive Fragmentation) as well as shrapnel projectiles with time fuzes.

The T-90S gun can also fire the 9M119 Refleks (NATO designation AT-11 Sniper) anti-tank guided missile system. The range of the missile is 100 - 4,000 m and takes 11.7 sec to reach maximum range. The system is intended to engage tanks fitted with ERA (Explosive Reactive Armour) as well as low-flying air targets such as helicopters, at a range of up to 5 km. The missile system fires either the 9M119 or 9M119M missiles which have semi-automatic laser beamriding guidance and a hollow charge warhead. Missile weight is 23.4 kg. The guns automatic loader will feed both ordnance and missiles.

Also fitted is a coaxial 7.62 mm PKT machine gun and a 12.7 mm air defence machine gun. A 5.45 mm AKS-74 assault rifle is carried on a storage rack.

The T-90 tank is protected by both conventional armour-plating and explosive reactive armour (ERA).

The T-90 is fitted with the Shtora-1 defensive aids suite which is produced by Electronintorg of Russia. This system includes infrared jammer, laser warning system with four laser warning receivers, grenade discharging system which produces an aerosol screen and a computerised control system.

It is also fitted with NBC (nuclear, biological and chemical) protection equipment.

The T-90S has the 1A4GT integrated fire control system (IFCS) which is automatic but with manual override for the commander. The IFCS contains the gunner's 1A43 day fire control system, gunner's TO1-KO1 thermal imaging sight which has a target identification range of 1.2 km to 1.5 km and commander's PNK-S sight.

The gunner's 1A43 day FCS comprises: 1G46 day sight/rangefinder with missile guidance channel, 2E42-4 armament stabiliser, 1V528 ballistic computer and DVE-BS wind gauge.

The commander's PNK-4S sight includes a TKN-4S (Agat-S) day/night sight which has identification ranges of 800 m (day) and 700 m (night).

The driver is equipped with a TVN-5 infrared night viewer.

The T-90S has a liquid-cooled V-84MS 618 kW (840 hp) four-stroke V-12 piston engine. This engine can be fuelled by T-2 or TS-1 kerosene and A-72 benzine, in addition to diesel. The tank can carry up to 1,600 litres of fuel in the main fuel tanks and fuel drums. The fuel tanks are reinforced with armour plating.

The tank is provided with a snorkel for deep fording and can ford 5 metres of water with equipment which can be deployed in 20 minutes.

The mechanical transmission includes primary reduction gear, two planetary final gearboxes and two planetary final drives. The running gear features torsion bar suspension with hydraulic shock absorbers at 1, 2 and 6 road wheel stations and tracks with rubber-metallic pin hinges.