India is set to significantly strengthen its strategic defense with the induction of two advanced S4-class nuclear ballistic missile submarines, INS Aridhaman and its sister ship, by early 2027. Building on the success of earlier Arihant-class submarines, these vessels represent a major leap in India’s second-strike nuclear capability, a core pillar of its credible minimum deterrence policy. INS Aridhaman, launched in November 2021, is undergoing final sea trials and is expected to join the Indian Navy by the end of 2025, with its sister vessel slated for commissioning by early 2027. These submarines feature larger 7,000-tonne hulls, approximately 1,000 tonnes heavier than predecessors, equipped with upgraded 83-megawatt pressurized water reactors enabling extended 90-day patrols without surfacing. Increased missile capacity with up to eight vertical launch tubes enables carriage of longer-range K-4 submarine-launched ballistic missiles with 3,500 km strike range, substantially enhancing India’s ability to execute a retaliatory nuclear strike even if under first attack. The vessels are built with around 75% indigenous content, marking India’s growing self-reliance in critical defense technology. Together, these submarines will fortify India’s deterrence posture amid rising regional security challenges.