Congress Legislature Party leader Dharam Singh will be the new chief minister of Karnataka and Janata Dal-Secular leader Siddaramaiah will be his deputy.
The parties reached this agreement to form a coalition government in the state, where assembly polls produced a fractured verdict.
Singh said he would take the oath of office and secrecy on Friday.
Ahmed Patel, political secretary to Congress president Sonia Gandhi, made the announcement about the power-sharing agreement after a day of hectic negotiations.
During the day, Singh met Gandhi while JD-S president and former prime minister H D Deve Gowda held consultations with Siddaramiah.
Gowda then met Singh and his party colleagues Ahmed Patel and Vilasrao Deshmukh to break the logjam.