The Tamil Nadu assembly election will be held in a single phase on April 6.
The DMK has allotted 25 seats to ally Congress for next month's elections in Tamil Nadu. The national party has also been allotted the Kanyakumari Lok Sabha seat that would go for bypolls following the death of Congress MP Vasanthakumar.
"The DMK and the Congress have sealed seat-sharing deal. The Congress will contest in 25 assembly seats and the Kanyakumari Lok Sabha seat. Both cadre will now work together. We will win big," Dinesh Gundu Rao, the Congress leader in-charge of Tamil Nadu, told NDTV.
The DMK will contest in 180 seats, he said. The MK Stalin-led party has also not given assurance on the Rajya Sabha seat, he said.
Mr Rao, who visited DMK headquarters this morning to finalise the seat-sharing deal, said he was confident that the alliance will sweep the state polls.
"BJP by entering into an alliance with AIADMK, its intention is to finish off AIADMK. They want to kill all opposition parties and have 'one party-one man rule' in the country. Congress, DMK, Left, VCK will sweep these elections," Mr Rao was quoted by news agency ANI after announcing the deal.
The Congress share for the assembly election is a significant climbdown from the 41 seats it contested in the 2016 polls. It had won just eight then.
According to sources, the Tamil Nadu Congress had asked for at least 30 seats while the DMK said it would only spare 24, leading to a stalemate. The deal was finalised after the Congress leadership in Delhi got in touch with the DMK's top leadership in Chennai on Saturday evening, they said.
With little bargaining power in the state, Tamil Nadu Congress chief KS Alagiri had said last week the "ball is the DMK's court" when it came to seat-sharing for the elections.
There have been multiple rounds of talks between the two sides on a seat-sharing pact while Congress chief Rahul Gandhi made several trips to the state to campaign for the polls.
In the 2016 assembly polls, the Congress was allotted 41 seats and it had won eight of them.
"The ball is in their (DMK's) court. Hence, it is the DMK which has to decide (seat-sharing)," Mr Alagiri had said when asked about how many seats his party had requested.
The DMK, hoping to return to power after being in opposition for 10 years, has allotted six seats to VCK, MDMK AND CPI, three seats to the Indian Union Muslim League and two to Manithaneya Makkal Katchi. It is also holding talks with the CPM.
The party's decision on the Congress comes a day after its rival AIADMK allotted 20 seats to the BJP.
The Tamil Nadu assembly election will be held in a single phase on April 6 and the counting of votes will be done on May 2.
Tamil Nadu will see a contest mostly between the ruling AIADMK and opposition DMK while many will also keep their eyes peeled for how actor-turned-politician Kamal Haasan's new party Makkal Needhi Maiam fares.
This is the first election after the death of two iconic leaders J Jayalalithaa of the AIADMK and M Karunanidhi of the DMK.