"Not proved, therefore not guilty," the Pennsylvania Republican said.
Initially, there was a pause among Senate clerks who seemed unsure how to record such a vote. The impeachment rules say a senator can vote only guilty or not guilty.
But when Chief Justice William Rehnquist announced the vote, Specter was counted in the not guilty column.
Specter, a former Philadelphia prosecutor, had announced earlier this week he planned to vote using these words based on Scottish court practice that allowed the nuance of "not proved" as an alternative verdict. Specter said he did not necessarily believe Clinton was innocent but that the House had not had a fair chance to prove its case.