"Final talks with Congress have been completed. We are going to declare the alliance and seat-sharing in all probability this evening," Prasad told a public meeting here.
"List of candidates for the elections will be declared within two days," he said.
RJD sources said both parties have agreed to "concede" to each other's demands.
They also said the alliance was arrived at with the help of outgoing Kerala governor Nikhil Kumar, whose house Prasad visited in Patna yesterday.
Earlier, Prasad had offered 11 seats to Congress and one to Nationalist Congress Party (NCP) as a pre-condition for alliance. There were contentions on a few constituencies as well.
Prasad addressed back-to-back public rallies at Fatehpur and Guraru in Gaya district during the day.