Prakash Karat was on Monday unanimously elected general secretary of the Communist Party of India-Marxist.
Karat, 56, takes over from Harkishen Singh Surjeet, who offered to step down from the post.
Karat's wife, Brinda, became the first woman entrant to the all-male politburo of the party.
The election of Karat, who hails from Kerala, came at the end of the six-day 18th congress of the party.
An ageing Surjeet, now 89, had earlier offered to step down from the post. Along with him, the 91-year
Both leaders were, however, persuaded to stay on.
The new politburo comprises Harkishen Singh Surjeet, Jyoti Basu, Prakash Karat, V S Achuthanandan, S Ramachandran Pillai, Sitaram Yechury, R Umanath, M K Pandhe, Biman Basu, Anil Biswas, Tripura Chief Minister Manik Sarkar, Pinarayi Vijayan and West Bengal Chief Minister Buddhadeb Bhattacharjee.
The four new members elected to the politburo are Chittabrata Majumdar, K Varadarajan, B V Raghavulu and Brinda Karat.