We examine the impact of proprietary and agency cost motives on segment disclosure quality and quantity and how the adoption of the principle IFRS 8 affects this impact. By using hand-collected data, our results show that proprietary and agency costs play a relevant role in determining the quality and quantity of segment disclosure. We find that proprietary costs are a particularly relevant reason for providing lower segment disclosure quality post-IFRS 8. Our results also suggest that firms’ segment disclosure choice is dependent on disclosure dimension. These results contribute to the ongoing debate regarding IFRS 8 and have valuable implications for accounting regulators.