Medvedev, who became a member of United Russia May 22, has proposed party reforms he said would make it more efficient and transparent, RIA Novosti reported.
United Russia won a narrow majority in the State Duma, the lower house of Parliament, in December elections that drew widespread fraud allegations.
The party, which some critics have called "the party of crooks and thieves," had a 50 percent support rating in May, state-run pollster VTsIOM said.
President Vladimir Putin, who had headed United Russia since he became prime minister in 2008, announced in late April he was stepping down as leader of the party.
Before officially taking the helm, Medvedev said the party should rely on secret ballots to pick its candidates for speaker of the State Duma, the lower house of Parliament, and leaders of party factions and branches as well as regional legislatures and governor vacancies.
He also said federal and regional party leaders should face election every five years.
Medvedev said United Russia also should have debates with political opponents. The party has been reluctant to do so, he said.
"In the future parliamentary election, United Russia will again try to win the support of the majority of citizens," he said.
Medvedev said the "absolute majority of party members and people sympathizing with the party are honest and decent. But there is nothing extraordinary in the fact that some crooks are constantly trying to hang on.
"We must get rid without delay of those who discredit themselves and thus discredit the party and instead attract to party ranks as many new, sincere, creatively-thinking people with initiative as possible."
Interpol investigating stolen passports on missing flight
Jessica Simpson shares three-way kiss with friends in photo