The Arihant-class (transl. 'Slayer of Enemies' in Sanskrit) is a class of Indian nuclear ballistic missile submarines under construction for the Indian Navy. They were developed under the ₹900 billion (US$11 billion) Advanced Technology Vessel (ATV) project to design and build nuclear-powered submarines. These vessels are classified as 'strategic strike nuclear submarines' by India.

The lead vessel of the class, INS Arihant was laid down in 2004, launched in 2009 and after extensive sea trials was confirmed to be commissioned in August 2016. Arihant holds the distinction of being the first ballistic missile submarine to have been built by a country other than one of the five permanent members of the United Nations Security Council. As of 25 October 2024, INS Arihant and INS Arighaat are already on deep sea patrols.

In December 1971, during the Indo-Pakistani War of 1971, US President Richard Nixon sent a carrier battle group named Task Force 74, led by the nuclear-powered USS Enterprise into the international waters of the Bay of Bengal in a show of force. Task Force 74 remained in international waters, where it was legally entitled to be. The records of Nixon-Kissinger communications show no contingency nor any plan under which Enterprise would enter Indian or Pakistani waters, or otherwise intervene in the conflict but it is well known that the duo viewed Pakistan as a strong ally in that region and were silent on Bangladesh genocide committed by Pakistani military. In response, the Soviet Union sent a submarine armed with nuclear missiles from Vladivostok to trail the US task force in support of India. The event demonstrated the significance of nuclear weapons and ballistic missile submarines to then Prime Minister Indira Gandhi. Following the 1974 Smiling Buddha nuclear test, the Director of Marine Engineering (DME) at Naval Headquarters initiated a technical feasibility study for an indigenous nuclear propulsion system (Project 932).

The Indian Navy's Advanced Technology Vessel project to design and construct a nuclear submarine took shape in the 1990s. Then Defence Minister George Fernandes confirmed the project in 1998. The initial intent of the project was to design nuclear-powered fast attack submarines, though following nuclear tests conducted by India in 1998 at Pokhran Test Range and the Indian pledge of no first use, the project was re-aligned towards the design of a ballistic missile submarine in order to complete India's nuclear triad.

The Arihant-class submarines are nuclear powered ballistic missile submarines built under the Advanced Technology Vessel (ATV) project. They will be the first nuclear submarines designed and built by India. The submarines are 111 m (364 ft) long with a beam of 11 m (36 ft), a draught of 15 m (49 ft), displacement of 6,000 tonnes (5,900 long tons; 6,600 short tons). The complement is about 95, including officers and sailors. The boats are powered by a single seven blade propeller powered by an 83 MW pressurised water reactor and can achieve a maximum speed of 12–15 knots (22–28 km/h) when surfaced and 24 knots (44 km/h) when submerged.

The submarines have four launch tubes in their hump and can carry up to twelve K-15 Sagarika missiles with one warhead each (with a range of 750 km or 470 mi) or four K-4 missiles (with a range of 3,500 km or 2,200 mi). The third and fourth submarines will have a larger configuration, carrying twenty-four K-15 Sagarika or eight K-4 missiles. The Indian Navy will train on INS Chakra, an Akula-class submarine leased from Russia in 2012.

The submarine's K-15 missiles can reach most of Pakistan and its K-4 can target all of Pakistan. The K-4 may also be capable of targeting Beijing, but would need to be in the northern most waters of Bay of Bengal. Deployment of the Arihant to the Pacific Ocean is unlikely given the submarine's noise issues.

The submarines are powered by a pressurised water reactor with highly enriched uranium fuel. The miniaturised version of the reactor was designed and built by the Bhabha Atomic Research Centre (BARC) at the Indira Gandhi Centre for Atomic Research (IGCAR) in Kalpakkam. It included a 42-metre (138 ft) section of the submarine's pressure hull containing the shielding tank with water and the reactor, a control room, as well as an auxiliary control room for monitoring safety parameters. The prototype reactor became critical on 11 November 2003 and was declared operational on 22 September 2006. Successful operation of the prototype for three years enabled the production version of the reactor for Arihant. The reactor subsystems were tested at the Machinery Test Center in Visakhapatnam. Facilities for loading and replacing the fuel cores of the naval reactors in berthed submarines were also established. The prototype 83 MW light water reactor that was installed at Kalpakkam by BARC is codenamed S1 and is used to train nuclear submariners.

In 2007, then finance minister P. Chidambaram, who was a member of the political committee which monitors the ATV programme, questioned the huge amount of money being spent on submarines with just 4 missile launch tubes. Hence, the ATV project team tweaked the Arihant design by adding a 10-metre-long section for four more K-4 SLBMs to be integrated into the boat codenamed S4. After it became evident that the larger S5 class of SSBNs will take more time to develop, an additional unit, codenamed S4*, was sanctioned in 2012 to ensure that the production line doesn't go idle. Hence, the final two ships INS Aridhaman (S4) and S4* in the class are expected to be larger, displacing over 1,000 tonnes more than the Arihant and have 8 missile launch tubes to carry up to 24 K15 or 8 K4 or K5 and a more powerful pressurized water reactor than INS Arihant.

The detailed engineering of the design was implemented at Larsen & Toubro's submarine design centre at their Hazira shipbuilding facility. Tata Power SED built the control systems for the submarine. The steam turbines and associated systems integrated with the reactor were supplied by Walchandnagar Industries. The lead vessel underwent a long and extensive process of testing after its launch in July 2009. The propulsion and power systems were tested with high-pressure steam trials followed by harbor-acceptance trials that included submersion tests by flooding its ballast tanks and controlled dives to limited depths. INS Arihant's reactor went critical for the first time on 10 August 2013. On 13 December 2014, the submarine set off for its extensive sea trials. The first boat of the class, INS Arihant, was commissioned in August 2016. On 5 November 2018, INS Arihant completed its first deterrence patrol.

In December 2014, the work on a second nuclear reactor began and INS Arighaat started sea trials in 2017. As per reports released in August 2024, INS Arighaat was commissioned before October 2024. On 29 August 2024, after extensive trials and upgrades INS Arighaat was formally commissioned.

INS Aridhaman (S4) was launched in November 2021. On 30 August 2024, a report suggested that this submarine will be commissioned in the next 6 months. In August 2024, it was reported that a fifth Arihant-class submarine which will be the third of S4 subclass is to be constructed.

The fourth submarine of the class, with an indigenous content of 75% and equipped with K4 missiles, was launched on 16 October 2024.

Name	Pennant	Displacement (surfaced)	Laid down	Launch	Sea Trials Begins	Sea Trials Completed	Commission	Status
INS Arihant (S2)	SSBN 80	6,000 tonnes	2004	26 July 2009	13 December 2014	23 February 2016	August 2016	Active
INS Arighaat (S3)	SSBN 81		2009	19 November 2017	8 January 2018	March 2021	29 August 2024
INS Aridhaman (S4)		7,000 tonnes		23 November 2021	N/A		February 2025 (est.)	Sea Trial
S4* (codename)				16 October 2024				Launched

Date	Event
19 May 1998	Confirmation of ATV project by the then Defence Minister George Fernandes
11 November 2003	Prototype nuclear reactor becomes critical
22 September 2006	Nuclear reactor is declared operational
2007	Design of S4 is modified to allow four more launch tubes.
26 July 2009	Lead vessel of the class, INS Arihant, is formally launched
2012	Additional unit S4* is cleared to avoid idling of production line.
10 August 2013	Arihant's on-board nuclear reactor attains criticality
13 December 2014	INS Arihant begins extensive sea & weapons trials
25 November 2015	INS Arihant successfully test-fired dummy B5 missile
31 March 2016	INS Arihant successfully test-fired K4 missile
August 2016	INS Arihant commissioned.
19 November 2017	INS Arighaat launched
8 January 2018	INS Arighaat to begin sea trials
5 November 2018	INS Arihant completed its first deterrence patrol
March 2021	INS Arighaat's sea trials are completed.
November 2021	S4 is speculated to be launched.
29 August 2024	INS Arighaat officially commissioned.
16 October 2024	S4* launched
February 2025	S4 commissioning expected

• Future of the Indian Navy
• List of active Indian Navy ships
• List of submarines of the Indian Navy
• List of submarine classes in service
• Submarine-launched ballistic missile

• The Indian Strategic Nuclear Submarine Project by Mark Gorwitz