Data from the National Center for Health Statistics showed a drop for men from 76.5 years in 2014 to 76.3 in 2015, and from 81.3 to 81.2 for women. The preliminary figures show rises in several causes of death, especially heart disease, dementia and accidental infant deaths.
Life expectancy last fell during the peak of the HIV/Aids crisis in 1993. It has improved slightly in most of the years since World War Two, rising from a little more than 68 years in 1950.
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It also fell in 1980, after a severe outbreak of flu.
Overall life expectancy for men and women is now 78.8 years, a decrease of 0.1 year from 2014.
"This is unusual," lead author Jiaquan Xu, an epidemiologist at the NCHS, told AFP news agency. "2015 is kind of different from every year. It looks like much more death than we have seen in the last few years."