More farmers have been trying to grow the crop as the price of opium has been rising, it said.
The fall in the harvest is being attributed to bad weather and disease, the BBC reports.
The UNODC reported that the rise in the area devoted to growing opium poppies came despite the intensification of government eradication efforts, with almost 10,000 hectares of the crop eradicated in 2012.
"Cultivation was concentrated in the south and west of the country in areas where insecurity and organised crime are present", UNODC said.
The number of provinces with 'poppy-free' status remain unchanged at 17, the report added.
According to the report,Afghanistan produces over 90 percent of the world's opium.