The Ivey Business School Purchasing Managers Index (PMI), measuring Canada's economic activity, fell to 51.9 in December from an unrevised 60.0 in November.
Economists had expected the gauge to hit 53.8.
A figure above 50 shows an increase while below 50 shows a decrease.
Within sub-indexes, the inventories indicator dropped to 52.5 in December from 59.1 in the prior month, while the supplier deliveries gauge decreased to 49.3 from 52.7. At the same time, the prices index climbed to 59.5 in December from 54.9 in November and the employment measure rose to 51.4 from 50.3.