Tharoor filed his papers before District Collector Biju Prabhakar. The seat is likely to witness a three-cornered fight among sitting MP Tharoor, CPI's Bennet Abraham and O Rajagopal of BJP.
Abraham, the LDF candidate, is yet to file his papers.
Rajagopal is a former Union minister.
In 2009, Tharoor had won the seat by defeating CPI's P Ramachandran Nair by a margin of nearly one lakh votes.
Another prominent candidate to enter the fray in Kerala today was K P Dhanapalan of Congress in Thrissur. Dhanapalan, who won from the nearby Chalakkudi last time, has swapped the constituency for senior leader P C Chacko this time.