Exit polls reported on Romanian television showed voters rejecting Basescu's dismissal by some 75 percent or more.
Official results were expected to be released later.
Turnout in the referendum was reported relatively low, at just some 34 percent of Romania's 18 million voters.
Romania's parliament suspended Basescu on April 20, accusing him of violating the constitution, despite a Constitutional Court ruling that the charges were groundless.
Basescu says his opponents initiated the impeachment process to stop his pro-reform and anti-corruption drive.
Basescu has been locked in a long-running, bitter power struggle with liberal Prime Minister Calin Popescu Tariceanu, a former ally.
Basescu has accused Tariceanu of shielding corrupt politicians and the new business elites that enriched themselves during the transition from communism to a market economy.
Tariceanu says Basescu has an autocratic style and a "thirst for power."
Tariceanu said after casting his ballot that he voted for Basescu's impeachment "hoping to prevent a single person ... from monopolizing the whole political power in Romania."
(compiled from agency reports)