Mahendran's candidature was decided at a meeting of the party's State Committee attended by its National Secretary D Raja and Tamil Nadu secretary R Mutharasan.
Raja, who announced Mahendran's candidature, said that the party, with the support of CPI(M), was contesting the elections to "uphold parliamentary democracy" even as others including DMK and PMK had decided to boycott it.
"The Communist parties contesting this election will have a big impact in Tamil Nadu politics. We are contesting the bypoll to uphold parliamentary democracy, ideals of social justice and (strive) for corruption-free governance," he told PTI.
The two Left parties will focus on issues concerning the constituency such as housing and road connectivity besides others related to the state, he said.
R K Nagar seat fell vacant on May 17 after AIADMK's sitting MLA P Vetrivel resigned following the acquittal of Jayalalithaa on May 11 by the Karnataka High Court in the disproportionate assets case.
Jayalalithaa is contesting from the seat for her return to the state Assembly as an MLA. She had earlier filed her nomination today.