By-elections to nine state assembly constituencies were held in Tamil Nadu, in four separate phases. Election for Thirumangalam was held on 9 January and for Bargur, Thondamuthur, Ilaiyangudi, Cumbum, and Srivaikuntam constituencies on 18 August. Also, election took place for Vandavasi and Tiruchendur constituency on 19 December 2009 and finally for Pennagaram constituency on 27 March 2010. Dravida Munnetra Kazhagam (DMK) defeated All India Anna Dravida Munnetra Kazhagam (AIADMK) in Thirumangalam in the first phase and kept the winning momentum in the following general election.

Thondamuthur is a legislative assembly in Coimbatore district in Tamil Nadu, which includes the town, Thondamuthur. Its State Assembly Constituency number is 119. Thondamuthur assembly constituency is part of Pollachi Lok Sabha constituency. It was a part of Nilgiris Lok Sabha constituency till 2007. Election was not held in the year 1957. It is one of the 234 State Legislative Assembly Constituencies in Tamil Nadu, in India.