The Kremlin has given no indication about whether Putin will accept the role.
Turning down the chairman's position could suggest that Putin, after a trial period to make sure 42-year-old Medvedev settles into the Kremlin job, is planning to take a back seat.
Although Putin is expected to attend the second day of the party's conference on Tuesday, it was unclear whether he would use the occasion to accept or decline the role.
Delegates voted unanimously to create the post of chairman, which Gryzlov said would make Putin the leader of the party.
United Russia politicians said the party chairmanship would be a non-executive role that would give Putin overall strategic control of the party.
The president used a United Russia conference last year to announce that he could serve as premier once his presidency, limited by the constitution to two consecutive terms, came to an end.
The prime minister post is junior to the president, can be sacked at the president's order and often carries the blame for policy failures.
Putin filled the post with a series of low-level technocrats all seen as expendable.
Some analysts see a leadership role in United Russia as a way for Putin to preserve long-term influence by moulding the party, closely tied to the Kremlin since its creation, into a powerful political force in its own right.
While the president can sack the prime minister he has to seek the approval of parliament, controlled by United Russia, to appoint a new premier.
In a separate development, the Kremlin has announced that Putin will travel to Libya on Wednesday for a two-day trip, one of his last before stepping down.
In a statement, the Kremlin said: "On April 16 to April 17, 2008, Russian President Vladimir Putin will visit Libya at the invitation of ... Muammar Gaddafi."
Russian investment in Libya's gas and nuclear energy sectors are likely to be on the agenda as Moscow seeks to boost ties with the North African state that is emerging from 20 years of diplomatic and economic isolation.
Arms sales to Tripoli and the clearing of Soviet-era debt may also be touched on in talks between the two leaders.