The S&P Global Eurozone Composite PMI fell to 51.7 in October from September's 53.1, a four-month low, and under a preliminary estimate of 51.9, Reuters reported.
Readings above 50.0 indicate growth in activity.
"October's PMI survey data indicate that the euro zone economy is growing at a respectable though unexciting 0.3% rate at the start of the fourth quarter, building on the robust performance seen throughout the third quarter," said Chris Williamson, chief business economist at S&P Global Market Intelligence.
New orders grew only marginally in October, a steep drop from three months of stronger growth, with demand for services edging lower. Overall export orders rose for a third consecutive month but also lost pace. Both manufacturing and services output expanded at slower rates, pointing to a broad-based cooling.
Employment fell marginally. Services firms led the decline, partly negating a slight rise in factory headcounts.
Cost pressures intensified with input price inflation climbing to a five-month high. Businesses raised their charges sharply, though slightly less aggressively than in September.
"Further upward price pressures were reported in October, largely linked to higher energy prices, which are squeezing customers' budgets and discouraging spending," Williamson said.
As expected the European Central Bank left interest rates on hold last week, but economists polled by Reuters last month said it would hike again in December.
Business confidence weakened for a third straight month and remained below its long-run average.
France was the weakest performer, slipping back into contraction after a brief recovery in September. Germany, Italy, Spain and Ireland continued to grow, though all lost momentum.