Purple Line is a rapid transit metro line of the Kolkata Metro in Kolkata, West Bengal, India. A 7.75 kilometres (4.82 mi) stretch from Joka to Majerhat of this line is operational as of 2025. This line is planned to span 26.88 kilometres (16.70 mi) from IIM-Joka in the southern region of Greater Kolkata to Eden Gardens in the Central Kolkata. It will have interchanges with Blue Line at Park Street and Esplanade and with Green Line at Esplanade.

Previously, the stretch from Thakurpukur to Majerhat was surveyed as a branch line of the circular railway, and a metro line from Majerhat to Dakshineswar via Sealdah (interchange with Green Line) was planned. This plan was scrapped and a new metro line from further south in Joka to BBD Bagh was sanctioned in 2010–2011 with a total length of 17.22 km (10.70 mi) at an anticipated cost of ₹2,619.02 crore (equivalent to ₹59 billion or US$690 million in 2023). Later the route was truncated to Esplanade. The corridor runs along Diamond Harbour Road, Khidirpur Road, and Jawaharlal Nehru Road, major arterial roads of Kolkata, and has passenger interchange facilities with the Blue Line and Green line at Esplanade and Blue line at Park street. The proposed Esplanade station will not be the same as that of the Blue Line and Green Line but a different station that will be built at B.C. Roy market ground. The line now has a new depot in Joka. Due to land acquisition problems and objections from the Ministry of Defence, construction has been delayed several times since the beginning. Defence Ministry objected that the elevated corridor would overlook the Eastern Command headquarters at Fort William, Ordinance Depot at Mominpore. The change in alignment from elevated to underground increased the construction cost of the stretch from ₹139 crore (equivalent to ₹164 crore or US$19 million in 2023) to ₹3,000 crore (equivalent to ₹35 billion or US$420 million in 2023). The work resumed in several phases and new bids were invited by Rail Vikas Nigam Limited (RVNL) in April 2020. It is India's first metro line to run on indigenous head hardened rails, manufactured by Jindal Steel & Power. The extension of this line to IIM and Diamond Park for 2 km (1.2 mi) was sanctioned in the 2012–2013 Budget at a cost of ₹294.49 crore (equivalent to ₹555 crore or US$66 million in 2023). The work is being executed by RVNL.

The Mominpur metro station was planned to be built across a 2500 sq. m area. However, the Ministry of Defence objected to the elevated structure, saying that it would overlook the Ordnance depot. This forced RVNL to stall the entire project, and RVNL almost dropped the station from the plan even though it alone would have a projected 20,000 passengers during peak hours. Underground Mominpur station was also not possible due to the sharp gradient from Taratala metro station. After a series of discussions and consultations with the Ministry of Defence and Government of West Bengal in 2016, it was decided to shift the station around 1 km (0.62 mi) northward, near the Alipore Bodyguard Lines. But, after a year Defence Ministry approved the Mominpur metro station in its original location as the change in alignment would have delayed the project and budget overrun. It will be the last elevated station of the corridor. Now, the proposed underground Khidirpur metro station is planned at the Alipore Bodyguard Lines. There were also hurdles regarding clearance for tunneling under defence lands. In 2020, the Defence Ministry eased out the process as lease rent wasn't required anymore for tunneling as long the overground ownership of the land did not change. On 30 December 2022 the Joka - Taratala section and on 6 March 2024, the Taratala - Majerhat section was inaugurated remotely by Honourable Prime Minister Narendra Modi, completing the 7.75 km Phase 1 stretch.

On 12 January 2025, it was reported that Metro authorities decided to extend the line to Eden Gardens instead of Esplanade to facilitate commuters and connect Strand Road, BBD Bag and Calcutta High Court. Subsequently, in March 2025 railways sanctioned ₹1,000 crore (equivalent to ₹19 billion or US$220 million in 2023) for the extension.

On 25 March 2025, The Times of India reported that Metro railway sent a proposal to Ministry of Railways to extend the Diamond Park Branch to Tollygunge, which will be 8.5 km (5.3 mi) long. In this route three underground station at Thakurpukur Cancer Hospital, Ramchandrapur and Panchanantala will come.

Around 8.5 kilometres (5.3 mi) of the project will be elevated and the rest of 6.32 kilometres (3.93 mi) of the metro line will be underground. The project is getting delayed due to nonavailability of land for the construction of train depot at Joka. In 2019, RVNL was given the land required for building the depot and the construction for its boundary wall and land development started in February 2019. Tenders for its construction and plant & machinery were invited in April 2020.

Construction was further delayed in September 2018 due to the Majerhat Bridge collapse, the metro was being built right next to the bridge.

Tender for Majherhat to Esplanade underground section has been floated in 1st week of December 2019.