U.S. Secretary of State John Kerry last month said Japan and 10 EU countries qualified for exemptions to Iranian sanctions for moving away from Tehran's oil sector.
Japan's crude oil purchases as a whole were down 3.2 percent for March. Data from the Japanese Ministry of Finance show crude oil purchases for March from Iran dropped 175,000 barrels compared with last year, a 51 percent decline, Bloomberg News reports.
Crude oil purchases from Iran by Japan were 16 percent higher year-on-year for February, however.
Countries can get a waiver from U.S. sanctions targeting Iran's energy sector if they show "significant" cut backs in crude oil purchases.
Iranian candidates gearing up for June elections have put the economy at the top of their platform agenda. The Iranian currency collapsed last year under sanctions pressure.
Iranian officials said they were trying to move the economy away from a dependence on oil sales as a result of long-term sanctions pressure.