Boris Gryzlov, chairman of the party's supreme council and speaker of the lower house of parliament, said over 45,000 Agrarian party members had already submitted applications to join United Russia.
United Russia stated in April 2008 that it had 1.8 million members.
"This [merger] is a very important step in the strengthening of the Russian political system," Gryzlov said.
The two parties announced their plans to merge on September 12, and the Agrarian Party, which was founded in 1993, approved the decision at a meeting on October 10.
The left-wing Agrarian Party failed to win a single seat in the 2007 parliamentary elections, taking 2.3% of the vote.
Russian Prime Minister Vladimir Putin has headed the United Russia party ever since he stepped down as president in May.
