SPS announced Thursday that it had decided to dissolve at a congress on November 15, and had established an organizing committee to form a new democratic party along with two other liberal Russian political parties, which are broadly supportive of the Kremlin.
"I believe my participation in this project would not contribute to its success. That is why I have submitted a letter of resignation," Gaidar said in a statement.
Gaidar, an architect of economic reforms in post-Soviet Russia, was one of the founders of the SPS party.
Last week another founder and the former leader of the party, Nikita Belykh, also submitted a letter of resignation and was replaced by acting head Leonid Gozman.
The SPS, which advocates a free-market economy, performed well in the 1999 parliamentary elections, but gained less than 1% of the vote at the December 2, 2007, polls.