"We have stopped the 20 percent enrichment because we didn't need it (any more)," he was quoted as saying Tuesday by the semiofficial Fars News Agency.
Iran agreed to enrich uranium only to low levels under the terms of an interim agreement reached with Western powers in November. Iran secured modest relief from economic sanctions as part of the deal.
Araqchi said enrichment to 3.5 percent purity was ongoing.
Yukiya Amano, director general of the International Atomic Energy Agency, said Monday he couldn't determine if all of Iran's nuclear research was geared toward peaceful purposes. Enriching to 20 percent purity is viewed as a step toward a nuclear weapon.
Iran is suspected of developing the technology needed to manufacture a nuclear weapon, an allegation it denies.
Amano said, however, Iran had taken a "positive step forward" toward allaying international concerns, "but much remains to be done to resolve all outstanding issues."