After a high-level party meet, Vaiko said his party has unanimously decided to support the DMK in the by-election.
"It is a well thought-out decision considering the interests of Tamil Nadu and the Dravidian movement," he told reporters.
Asked if the alliance will continue, he said, "This can be taken as a starting point" in view of the cordial atmosphere between the two parties.
Vaiko further said, "There is a need to teach a lesson to the AIADMK regime which has lost the trust of the people."
He hit out at the BJP-led Centre over "Hindutva and weakening federalism" besides lashing out at the ruling party for hobnobbing with the saffron party.
DMK working president MK Stalin welcomed the decision, saying when all opposition parties were joining hands with it to end "this horse-trading regime," the MDMK has done its part.
The MDMK's decision to support the DMK comes after recent meetings between Vaiko and DMK working president M K Stalin.
Also, Vaiko recently had met DMK chief and former mentor M Karunanidhi at the latter's Gopalapuram residence here.
Karunanidhi, who was ill for about a year and confined to his residence is now recuperating following treatment.
The MDMK, which was part of the DMK-led alliance in Tamil Nadu, parted ways from it to support late J Jayalalithaa-led AIADMK in the 2006 Assembly election.
Vaiko had been detained under the now defunct Prevention of Terrorism Act by the then AIADMK regime in July 2002. After spending about 19 months in prison, he was released in February 2004.
In 2014, the MDMK was part of the BJP-led front whereas in 2011 the party quit the AIADMK-led alliance and also boycotted the Assembly election that year.
The MDMK fought the 2016 Assembly election as part of the People's Welfare Front, projected as an alternative to the two Dravidian majors, the AIADMK and the DMK.
In December 2016, the Vaiko-led party quit the combine which drew a blank.
The PWF comprised actor-turned politician Vijayakanth's DMDK, MDMK, CPI(M), CPI, VCK, and former Union minister GK Vasan's TMC(M).
In R K Nagar, DMK's N Marudu Ganesh is pitted against E Madusudanan of the ruling mainstream AIADMK.
Rival AIADMK leader TTV Dhinakaran is also in the fray.
The BJP has announced fielding its senior functionary Karu Nagarajan.
The DMK's allies Congress and IUML have already extended their support to the main opposition party.
Besides, parties including the CPI, CPI(M) and Viduthalai Chiruthaigal Katchi have declared their support to DMK.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)