Business confidence, often referred to as a leading indicator, turned positive for the first time since April 2011 to 0.7 points, from -2.8 points in May 2016.
Economists polled by Reuters had expected the index to decline slightly to -3.0 points.
The rise in confidence was strongest in manufacturing, with managers more favourable about their order books and stock levels.
The business-related services sector also saw an improvement, albeit to a lesser extent. Confidence among managers in the construction and trade industries declined slightly.
The index tends to be slightly positive at the top of an economic cycle and strongly negative at the depth of a recession.
Belgium is the first country to present business confidence figures this month, as Germany's closely watched Ifo index is due on Friday. Economists on average expect a small decline.