Mr Putin said: "We can say with certainty that the party has achieved a very good result."
The nationalist LDPR and the Communist party are way behind United Russia, with about 14-16% each.
Liberal opposition parties appear to have failed to pass the 5% threshold needed for party-list representation.
However, the exit polls say though they could still get seats in individual constituencies.
A VTsIOM exit poll gave United Russia, led by Prime Minister Dmitry Medvedev, 44.5%, with the Public Opinion Foundation putting its total higher, at 48.7%.