The move shall help set new caste equations for the Congress which is making an all-out bid to seize power from the BJP.
Hardik is not eligible for contesting elections due to his age.
He said the Congress has assured the PAAS that it would include its promise of reservation for the Patidars in the party manifesto.
Hardik said the promised quota would go beyond the 50 percent cap set by the Supreme Court for the Scheduled Castes, Scheduled Tribes, and OBCs.
"My fight in Gujarat is against the BJP and that is why we will directly or indirectly support Congress (in the elections) as it has accepted our demand for reservation," Hardik told reporters in Ahmedabad.
Under the proposal, the influential Patels, traditional voters of the BJP, would get the reservation in government jobs and educational institutions beyond the 49 percent quotas for the SC, ST, and OBCs.
Senior Congress leader Kapil Sibal thanked Hardik Patel for declaring his support to the party after it accepted the Patidar community's quota demand.
"We are very happy that a 'sanjha morcha' (united front) will now fight against the BJP... We thank him that they (PAAS) have joined us with an ideology to contest the election by coming together... Our target is to win the election and fulfil promises made to the people," Sibal told reporters in New Delhi.