The Revenue Service said Africa's biggest economy imported 505,908 tons of Iranian crude in March, up from 417,188 tons the previous month.
South Africa has come under Western pressure to cut Iranian crude imports as part of sanctions designed to halt Tehran's civilian nuclear program, and the increased crude imports from Iran show that it has rejected the Western demands.
South Africa's crude imports from Iran declined between October and January, but rose again in February.
Until late last year, Iran was typically South Africa's biggest crude supplier, accounting for a quarter of its oil imports.
According to the March data, crude imports totaled 1.6 million tons, with Nigeria supplying 38 percent, Iran 32 percent, Saudi Arabia 22 percent and Angola the rest.
The biggest South African buyer of Iranian crude used to be Engen, which is majority-owned by Malaysian state oil group Petronas. Petrochemicals group Sasol, BP, Shell , Total and Chevron are other potential buyers of Iranian crude.
Some South African refineries are designed to treat Iranian-type crude only, and refiners would be hard-pressed to replace those supplies with other products.
Any disruption in crude imports could hit fuel supplies in South Africa, which have already been volatile because of strikes and refinery problems.