The new figures confirm the view of central bankers that they can hold off on raising interest rates again any time soon in the absence of inflation pressure.
The Consumer Price Index, which tracks costs for household goods and services, fell by 0.1 percent last month from November, when it was unchanged, according to the Labor Department. It was the first decline since March and matched economists' expectations.
For 2018, the index rose a tepid 1.9 percent, dipping below two percent for the first time in 16 months.
The December energy index fell 3.5 percent, led lower by weakening gasoline and fuel oil prices, while food costs rose 0.4 percent.
Excluding the volatile food and fuel categories, core CPI rose 0.2 percent for the month. That also was in line with expectations and reflected higher costs for housing, home furnishings and medical care, according to the monthly report.
Core CPI held steady at 2.2 percent for the year.
After rising steadily in the first part of the year, inflation has been trending lower since July, confounding expectations that steady job growth, falling unemployment would at last drive prices higher.