Franklin County's unofficial results had Bush receiving 4,258 votes to Democrat John Kerry's 260 votes in a precinct in Gahanna. Records show only 638 voters cast ballots in that precinct.
Bush actually received 365 votes in the precinct, Matthew Damschroder, director of the Franklin County Board of Elections, told The Columbus Dispatch.
State and county election officials did not immediately respond to requests by The Associated Press for more details about the voting system and its vendor, and whether the error, if repeated elsewhere in Ohio, could have affected the outcome.
Bush won the state by more than 136,000 votes, according to unofficial results, and Kerry conceded the election on Wednesday after acknowledging that 155,000 provisional ballots yet to be counted in Ohio would not change the result. (Full Ohio results)
The Secretary of State's Office said Friday it could not revise Bush's total until the county reported the error.