Consumer prices dropped a seasonally adjusted 0.2 per cent in June from May, the Labor Department said.
The retreat in consumer inflation was much stronger than expected; the average analyst forecast was for a modest 0.1 per cent drop.
Gasoline prices led the decline in the energy price index, diving 6.8 per cent from May.
"While this decrease was the major factor in the seasonally adjusted decline in the all items index, the index for household energy declined as well," the Labor Department said.
Energy prices plunged 4.4 per cent last month, the steepest decline since December 2008.
Consumers also got some relief from rising food prices. The food index climbed 0.2 per cent, the smallest increase of the year.
Core inflation, which excludes food and energy prices that can be volatile month-to-month, rose 0.3 per cent for the second straight month.
Year-on-year, inflation was up 3.6 per cent and core inflation rose 1.6 per cent, its highest level since January 2010.
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