The sharp drop in profit was as a result of production drops and lower crude prices affecting its exploration and production division.
Net profits stood at €1.54 billion ($2 billion) for the three months to March, down from €3.61 billion in the year-ago period.
Eni has been confronted by a production drop of 4.9%, for a total of 1.6 million barrels of oil equivalent per day, due to “one-off” events in Nigeria, Libya and the UK.
Chief executive Paolo Scaroni acknowledged a “slower first quarter” but said the company would maintain its guidance numbers.
“Our E&P Division confirms its production growth targets for 2013 driven by continuing progress in developing ongoing projects,” he said in a statement.
Also affecting results was a 46% drop in earnings after “weak” performance from the engineering and construction division, which was “adversely affected by falling demand for oilfield services and lower margins at certain works.”