The Communist Party of India (Marxist) (abbreviated as CPI(M)) is a communist political party in India. It is the largest communist party in India in terms of membership and electoral seats, and one of the six national parties of India. The party was founded through a splitting from the Communist Party of India in 1964; the CPI(M) quickly became the dominant faction.

The 34 years of CPI(M)-led Left Front rule in West Bengal was the longest-serving democratically elected communist-led government in the world. It emerged as the third largest party of the parliament in the 2004 national election. Presently, CPI(M) is a part of ruling alliances in two states - the LDF in Kerala, which it leads, and the SPA in Tamil Nadu. It also has representation in the legislative assemblies of seven states.

The All-India Party Congress is the supreme authority of the CPI(M). However, during the time between two party congresses, the Central Committee is the highest decision-making body. The Central Committee shall elect from among its members a Polit Bureau including the General Secretary. The Polit Bureau carries on the work of the Central Committee between its two sessions and has the right to take political and organisational decisions in between two meetings of the Central Committee.

CPI(M) had a total income of ₹1,620,000,000 in fiscal year 2021–22. The party reported zero funding from electoral bonds.

CPI(M) is officially known as भारत की कम्युनिस्ट पार्टी (मार्क्सवादी) [Bhārat kī Kamyunisṭ Pārṭī (Mārksvādī)] in Hindi, but it is often known as मार्क्सवादी कम्युनिस्ट पार्टी (Mārksvādī Kamyunisṭ Pārṭī, abbreviated MaKaPa) in press and media circles. During its initial years after the split, the party was often referred to by different names such as 'Left Communist Party' or 'Communist Party of India (Left)'. The party has used the name 'Left' because CPI people were dubbed 'rightist' in nature for their support of the Congress-Nehru regime. During the Kerala Legislative Assembly elections of 1965, the party adopted the name 'Communist Party of India (Marxist)'. It also applied to obtain its election symbol from the Election Commission of India.

The Communist Party of India (Marxist) emerged from a division within the Communist Party of India. The CPI had experienced an upsurge in support during the years following the World War II; it had also led armed rebellions in Telangana, Tripura, and Kerala. However, it soon abandoned the strategy of armed revolution in favor of working within the Parliament framework. In 1950, B. T. Ranadive, the CPI general secretary and a prominent representative of the radical sector inside the party, was demoted on grounds of left-adventurism.

Under the government of the Indian National Congress party of Jawaharlal Nehru, independent India developed close relations and a strategic partnership with the Soviet Union. The Soviet government consequently wished that the Indian communists moderate their criticism towards the Indian state and assume a supportive role towards the Congress governments. However, large sections of the CPI claimed that India remained a semi-feudal country and that Class conflict could not be put on the back-burner for the sake of guarding the interests of Soviet trade and foreign policy. Moreover, the Indian National Congress appeared to be generally hostile towards political competition. In 1959 the central government intervened to impose President's rule in Kerala, toppling the E. M. S. Namboodiripad cabinet (the sole non-Congress state government in the country).

The basis of difference in opinion between the two factions in CPI was ideological – about the assessment of the Indian scenario and the development of a party programme. This difference in opinion was also a reflection of whether the Communist Party in India would toe the line of the Communist Party of the Soviet Union (CPSU) or follow an independent path based on the concrete analysis of the Indian situation. The alleged 'right-wing' inside the party followed the Soviet path whereas the 'left-wing' wanted to follow the mass party with a class line with national characteristics, based on the 'independent' development of socialism in accordance to the India situation. Moreover, the faction of CPI which later became CPI(M) referred to the "right" strategy as a national approach of class collaboration; this was a damning charge within the communist movement, in which the prioritization of working-class interests and independence is considered paramount. Ideological difference also grew on the analysis of the role and character of the Indian bourgeoisie and the character of the Indian revolution. While the 'right wing' in the party sought the Indian bourgeoisie to have a 'progressive' character and called for a national democratic revolution, the 'left wing' sought the character of the Indian bourgeoisie to be essentially reactionary and called for a peoples' democratic revolution. However as the 'left wing' grew, the Congress and the party's 'right wing' dubbed them as pro-Chinese and essentially made extensive efforts to incriminate them of committing 'anti-national' activities. This ideological difference later intensified, and ultimately gave rise to the establishment of CPI(M).