The grim milestone represents a significant spike from the previous single-day death toll record of 279. The Health Ministry also announced 4,251 new infections, pushing the total count to 534,630.
Fatalities have soared in recent weeks, as authorities struggle to contain the virus's spread months into the pandemic. Health officials say the capital, Tehran, has run out of intensive care beds.
The Islamic Republic has seen the worst outbreak in the Middle East with a death toll that topped 30,000 this week. The government has resisted a total lockdown to salvage its devastated economy, already weakened by unprecedented US sanctions.
As the death toll skyrockets, eclipsing the previous highs recorded in the spring amid the worst of its outbreak, authorities have started to tighten restrictions.
The government ordered shut recently reopened schools and universities, as well as museums, libraries and other public places in Tehran earlier this month, and imposed a mask mandate outdoors.
The timing of the pandemic has proved particularly difficult for Iran's economy. The Trump administration re-imposed economic sanctions on Iran after its unilateral withdrawal in 2018 from Tehran's nuclear accord with world powers.