DMK chief M Karunanidhi and Tamil Nadu Congress Committee president K V Thangkabalu signed the agreement, hammered out after hard bargaining by both sides for about a fortnight.
"Following discussions, it has been decided to share 63 seats with the Congress," the copy of the agreement released to the media said.
Later, Thangkabalu described the DMK-Congress combine as a "winning alliance."
The seat-sharing talks had run into rough weather with the DMK refusing to allot 63 seats and the Congress dead set on its demand, which had brought the seven-year-old alliance under serious strain over the weekend.
The stalemate continued till Tuesday, when the DMK finally relented to the Congress's demand for 63 seats.
The DMK and Congress had contested the 2004 and 2009 Lok Sabha polls together, besides the 2006 assembly elections.
Thangkabalu also said the election panels of the two parties, comprising Union ministers P Chidambaram and G K Vasan and Tamil Nadu Deputy Chief Minister M K Stalin on either side, would meet on Thursday to finalise the constituencies they will contest from.
"We will take good decisions," he said.