Purushothaman was sworn-in by Tripura High Court Chief Justice Deepak Gupta at a simple ceremony at Raj Bhavan here.
Purushothaman, an 86-year-old veteran Congress leader who served in his native state Kerala as a minister and the assembly speaker and was also elected to the Lok Sabha twice, was appointed Mizoram governor by then president Pratibha Patil in August 2011.
He replaced Devanand Konwar, who was transferred to Tripura from Bihar March 25 last year and completed his tenure Saturday.
Tripura Chief Minister Manik Sarkar, his cabinet colleagues, leaders of various political parties and army and police officials were present during the ceremony.