Syrians voted in an election expected to deliver an overwhelming victory for Assad but which his opponents have called a charade.
"Today's presidential election in Syria is a disgrace," State Department spokeswoman Marie Harf told a briefing in Washington.
"Bashar al-Assad has no more credibility today than he did yesterday," Harf added.
She also referred to "the disgusting photos of President Assad voting, acting like this is a real election."
Three years of war have killed more than 160,000 people in Syria.
"Detached from reality and devoid of political participation, the Assad regime's staged election today continued a 40-year family legacy of violent suppression that brutally crushes political dissent and fails to fulfill Syrians' aspirations for peace and prosperity."