Medvedev said he would "join the party's ranks," something Putin, now prime minister, has never done, RIA Novosti reported.
Medvedev will end his four years as president May 7 and is expected to be named prime minister as part of a power swap reached with Putin last year.
Meeting with top party officials at his residence, Medvedev said he had never been a liberal despite being called that.
"I have often been told 'You are a liberal,' but I can tell you frankly ... I am a man with conservative values," he said.
He criticized the party's burgeoning membership list of about 2 million, saying it should "cleanse itself" of people who joined because of "career ambitions."
Medvedev said a president should be a "party member."
"Otherwise we will not be able to explain to the people which political force is in power," he said.
Medvedev said leadership of United Russia is "a very serious responsibility" for him, Russia-based RT television news network reported. He said he relied on United Russia for political guidance during his presidency.
"It's also the issue of the development of the country: Where it would go, what it would be like, and what will be happening with us," Medvedev said.