We examine the relation between labor union strength and investment efficiency using comprehensive firm-level data of Korean listed companies. We find that the perceived underinvestment related to unionization documented in previous studies is attributable to a negative relation between union strength and investment in overinvesting firms. In fact, union strength is positively related to the level of investment in underinvesting firms. We further find that the relation between union strength and investment efficiency is more pronounced for chaebol firms where inefficient investments are more likely due to greater agency problems between the controlling and minority shareholders. Finally, we document that the investment has more positive value implications in firms with a stronger union. Our results suggest that unions play an important role as a non-financial stakeholder in curbing inefficient investments.