Deepak Kumar (55) was admitted to a Gurgaon hospital on outskirts of Delhi for treatment where he died, party sources said.
Kumar was a four-time MLA from Unnao in Uttar Pradesh assembly.
Chief Minister Akhilesh Yadav and Samajwadi Party supremo Mulayam Singh Yadav have expressed profound grief over the MLA's demise.
In a statement, Akhilesh Yadav while terming Deepak Kumar as a "true socialist" said that he was always sensitive towards the problems of the poor and deprived and ever eager to solve them.
SP's legislature party meeting which was scheduled at the party headquarter here today was postponed because of the Kumar's death after a condolence meeting.