Sales of food, drinks and tobacco were down 7.1% on the year in 2011 in data adjusted for the number of working days and seasonal factors.
Fuel sales fell 1.8%, but non-food product sales inched up 0.2% in 2011.
In December alone, adjusted retail sales advanced 0.3% compared with November, fueled by increases in non-food product and fuel sales by 1.2% and 0.7% respectively, INS said.
Food sales fell 0.7% on the month in December 2011.
Compared with the same month of 2010, Romanian adjusted retail sales rose 3.9% in December.
Fuel sales increased 16.4% on the year in December, while non-food product sales were up 10.4%. Food sales fell 2.2% in December 2011 compared with the year-earlier month, the data showed.