Statistics agency Eurostat said consumer prices across the region were down 0.2 per cent in February from the year before, against a 0.3 per cent rise the previous month.
The decline was way more than anticipated -- the consensus in the markets was for a drop to zero.
The decline is largely due to a big decrease in energy costs, which were 8 per cent lower in the year to February against the previous month's 5.4 per cent drop.
Since the ECB aims for inflation just below 2 percent, February's negative rate could mean it cuts interest rates further or expands its bond-buying programme -- inflation has been below target since February 2013.
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"Poorly anchored inflation expectations and the cooling economy will prompt the ECB to ease already-accommodative monetary policy," said Tomas Holinka, economist at Moody's Analytics.
The eurozone experienced a bout of falling prices about a year ago but it didn't last long, helped in part by ECB stimulus measures and a falling euro, which made the price of imports dearer.