It was due to a persistent downturn in the bloc's dominant services industry.
That's according to a key survey released Thursday, which indicated the euro zone economy was in recession.
The closely watched Purchasing Managers' Index - or PMI - was revised up for December to match November's 47.6.
While that was an improvement on a preliminary reading, it was below the 50 mark for a seventh month.
Anything above 50 represents growth, but below that means contraction.
It indicated the euro zone likely shrank last quarter, following on from a 0.1% contraction in Q3 last year.
And that meets the technical definition of a recession.
December's services PMI slightly rose to a five-month high of 48.8 from November's 48.7.
A similar survey released Tuesday found euro zone factory activity contracted in December for an 18th straight month.
But overall sentiment about the year ahead improved as the composite future output index went up to a seven-month high.
