The Sentix research group said its investor sentiment index for the euro zone dropped to -16.8 in October from -11.1 in September. That was its lowest level since April 2013 and undershot the Reuters consensus forecast for a reading of -13.0.
"There hasn't been a positive reaction to the support measures taken by central banks, with economic assessments falling in October on a broad front," Sentix Managing Director Patrick Hussy said.
In September, European Central Bank chief Mario Draghi pledged indefinite stimulus to revive an ailing euro zone economy. The ECB cut rates deeper into negative territory and promised bond purchases with no end-date to push borrowing costs even lower.