Exempting seven economies from the stringent sanctions set to go into effect June 28 "underscores the success of our sanctions implementation," Clinton said in a statement.
India, Malaysia, South Africa, South Korea, Sri Lanka, Taiwan and Turkey received the exemptions Monday.
Financial institutions in those countries faced being cut off from the U.S. banking system if the countries do not reduce their imports enough before June 28 to be given a six-month exemption from the sanctions.
The countries weaned themselves enough to meet U.S. requirements, Clinton said.
"By reducing Iran's oil sales, we are sending a decisive message to Iran's leaders: Until they take concrete actions to satisfy the concerns of the international community, they will continue to face increasing isolation and pressure," Clinton's statement said.
The countries join Japan and 10 European nations -- Belgium, Britain, the Czech Republic, France, Germany, Greece, Italy, the Netherlands, Poland and Spain -- in being exempted.
Washington determined in March the initial group had "significantly reduced" Iranian oil purchases, qualifying them for the renewable 180-day exemption under a sanctions law President Barack Obama signed in December.
Cutting back on Iranian oil imports should not hurt these countries, the White House said.
"There currently appears to be sufficient supply of non-Iranian oil to permit foreign countries to significantly reduce their imports of Iranian oil," White House spokesman Jay Carney said in a statement.
Many of Iran's customers that cut back on Iranian crude "are in productive discussions with alternative suppliers," his statement said.
Iranian oil imports have dropped to 1.2 million to 1.8 million barrels a day from about 2.5 million barrels a day last year, the International Energy Agency estimates.
Clinton's exemption announcement came a week ahead of a third round of negotiations -- this time in Moscow -- between six world powers and Iran over its nuclear program.
The powers -- the United States, China, Russia, France, Britain and Germany, known by the shorthand P5-plus-1 because they're the five permanent U.N. Security Council members plus Germany -- seek to persuade Iran to cut back on its nuclear program out of concerns it seeks to develop nuclear weapons.
Iran says its activities are for peaceful civilian energy and medical purposes.
China -- historically Iran's biggest buyer of oil -- was not included in Clinton's exemption announcement, making it the only major Iranian oil importer not exempted from the U.S. sanctions.
Not exempting China doesn't mean Washington and Beijing are on a collision course, U.S. administration officials said.
Beijing has "been committed to working with us to help Iran from acquiring a nuclear weapon," a senior administration official said Monday. "It has shown that it's committed to a dual-track approach of both engagement and pressure, including sanctions."
China is "a very important partner in the P5-plus-1 process. It's been committed to working with us to help Iran from acquiring a nuclear weapon," the official said.
China voted for U.N. sanctions against Iran, even as it expressed reservations about the sanctions' effectiveness, said the official, one of two senior administration officials speaking with reporters on condition of anonymity.
Washington remains engaged in a "good-faith dialogue" with Beijing "to work toward a solution" to "reduce the volume of purchases of Iranian crude oil," one of the officials said, adding it would be "premature to comment further on where those discussions might lead."
Obama is authorized to issue a waiver to China before June 28 based on national security considerations, administration officials said.
China sharply reduced its oil imports from Iran in the first three months of 2012, but this was due to a price dispute, The New York Times reported.
Beijing's purchases spiked again in April, and the upswing carried over into June, the newspaper said.