The poll will decide the fate of 11 candidates, but all focus is on AIADMK's P. Saroja and DMK's V. Maran. All others are independents.
To ensure tight security, around 2,500 police personnel have been deployed in the hill station constituency.
The by-election was declared after AIADMK legislator C. Perumal died in July. Saroja is Perumal's wife.
Death of Perumal led to the AIADMK's strength in the 235-member assembly come down to 150. The DMDK is the second largest party in the house with 29 members, followed by the DMK with 23.
The total number of voters in Yercaud constituency is around 240,000 of which men constitute around 119,000 and women around 121,000.
This will be the fourth assembly by-election in the state after the AIADMK came to power in May 2011. The AIADMK won all three earlier elections at Pudukottai, Sankarankoil and Tiruchirapalli (West) constituencies.
By-poll in Surat West assembly in Gujarat
Voting also took place for the bypoll in Surat West Assembly constituency in Gujarat, where 24 candidates are in the fray. The bypoll was declared following an untimely death of two time sitting BJP MLA Kishore Wankawala. Over 54 per cent voter turnout was recorded in Surat West assembly.
(With agency inputs)