Statistics Canada says manufacturing sales fell 1.0 per cent to $49.9 billion in May.
Economists had expected a drop of 0.7 per cent, according to Thomson Reuters.
The decline came as sales of motor vehicles and petroleum and coal products dropped.
Motor vehicle sales fell 4.2 per cent to $5.6 billion, while sales of motor vehicle parts declined 2.3 per cent.
Sales in the petroleum and coal products industry fell 2.2 per cent to $4.1 billion.
Sales fell in 15 of 21 industries, representing nearly 70 per cent of total manufacturing.
Constant dollar sales fell 2.1 per cent, indicating that a lower volume of manufactured goods was sold in the month.