The 2021 West Bengal Legislative Assembly election was the 17th quinquennial legislative election held in West Bengal, to elect all 294 members of West Bengal Legislative Assembly. This electoral process of 292 seats unfolded between 27 March to 29 April 2021, taking place in eight phases. Voting for the two remaining constituencies was delayed to 30 September 2021.

The incumbent Trinamool Congress government led by Mamata Banerjee won the election by a landslide, despite opinion polls generally predicting a close race against the Bharatiya Janata Party, which became the official opposition with 77 seats. For the first time in the history of Bengal, no members from INC and Communist party were elected.

Outlined in Article 168 of the Constitution of India, the West Bengal Legislative Assembly is the only house of the unicameral legislature of West Bengal, not a permanent body, and subject to dissolution. The assembly term lasts for five years unless it is dissolved earlier. Members of the Legislative Assembly are directly elected by the people, and the tenure of the Sixteenth West Bengal Legislative Assembly was scheduled to end on 30 May 2021.

In the 2016 election, the All India Trinamool Congress (AITC or TMC) retained its majority in the Legislative Assembly with 211 seats. The Indian National Congress won 44 seats and the Left Front won 33 seats from their alliance, while the Bharatiya Janata Party and the Gorkha Janmukti Morcha won 3 seats each out of the total 294 seats.

Since the by-elections held for the Kanthi South seat in 2017, it became evident that the Bharatiya Janata Party (BJP) had overtaken the Left Front as the primary opposition party in the state. According to various political analysts, the shifting of the Left Front and other opposition voters towards the BJP caused the party's vote share to significantly increase. In spite of widespread violence, the BJP emerged as the second largest party in the 2018 elections to the state panchayats mainly due to the shifting of the Left Front's voter base. The long-held stereotype of Bengali Hindus being averse to right-wing politics was shattered when the BJP won the 2018 assembly elections in Tripura, another Indian state with a Bengali Hindu majority, ruled till then by CPI(M)-led alliance of Communist parties since 1993.

In the 2019 general elections, the BJP increased its number of Lok Sabha seats from 2 to 18, and took 40% of the vote share, an increase from 11% in the 2016 elections. Trinamool Congress (TMC) was reduced from 34 to 22 seats, Indian National Congress (INC) was reduced from 4 to 2 seats, and for the first time since their individual inceptions, no party from the Left Front (namely CPI(M), CPI, AIFB & RSP) was able to win a single seat from the state. This was the best ever performance of the BJP in the state (where it had never won more than 2 seats) in terms of both seats & voteshare. Public anger towards corruption and hooliganism of a section of TMC cadres in rural areas during the 2018 panchayat elections, religious polarisation by BJP fueled by resentment of a section of Bengali Hindu society towards Mamata Banerjee's tactics of Muslim appeasement, and large scale support of the Rajbongshi and Matua communities for granting Indian citizenship to exclusively non-Muslim Bangladeshi immigrants over fears of a demographic change fuelled by infiltration of undocumented Muslim immigrants from Bangladesh, allegedly supported by Mamata Banerjee have been cited as important reasons behind the rise of BJP in West Bengal alongside the decline of Left Front.

With Narendra Modi becoming the only non-Congress prime minister to remain in power for two consecutive terms (amounting to ten years) without depending on the support of the National Democratic Alliance, and the BJP fulfilling the wish of its founder Shyamaprasad Mukherjee by revoking the special status of Jammu and Kashmir and long standing promise of resolving the Ayodhya dispute in favour of Hindus, the party considered the formation of a BJP-led state government in West Bengal (a state which has historically never voted for right-wing parties in large numbers) for the first time as a means of paying homage to Mukherjee, who hailed from there. A BJP victory in West Bengal would have also demoralised Mamata Banerjee's attempts of creating a non-BJP non-Congress alliance of regional parties that might play an important role in the upcoming general elections.

BJP increased their seats in the assembly from 3 to 53 when the West Bengal Legislative Assembly was dissolved through defections from TMC, INC, and Left Front leaders, and by-elections from 2016 to 2021. A prominent defector in December 2020 was Suvendu Adhikari, who was a long-time associate of Mamata Banerjee, and a state cabinet minister who was dissatisfied over the rising influence of her nephew Abhishek Banerjee in the party. However, Adhikari revealed that he was in contact with the BJP since 2014 after he joined the party. His father Sisir Adhikari, the MP from Kanthi, also defected from TMC to BJP. Another cabinet minister, Rajib Banerjee, also joined BJP.