The central bank, which set a 2-4 percent goal for 2011, introduced the inflation targeting regime in 2005.
Consumer prices were up 0.2 percent on the month. Food prices, which have fallen sharply in recent months due to a better than expected harvest, were up 0.4 percent, the National Statistics Board said.
Non-food prices rose 0.1 and services were up 0.3 percent.
A Reuters poll of 10 analysts earlier this month had forecast inflation would slow to 3.3 percent year-on-year in December. The monthly forecast was 0.4 percent.
Central bank governor Mugur Isarescu had said December inflation could be below the central bank's forecast of 3.3 percent.
The bank cut interest rates by a quarter point to 5.75 for the second time in a row last week, taking advantage of slowing inflation to give a boost to slow economic recovery.