In this study, we examine the impact of labor union power on stock price synchronicity. We proxy labor union power by the collective relation law index and measure stock price synchronicity by the level of co-movement between a firm’s stock price and market index. Using a sample of 37 countries between 1995 and 2016, we find that labor union power is positively associated with stock price synchronicity, indicating that firms in countries where labor union power is relatively stronger tend to disclose less firm-specific information. Our evidence also shows that this phenomenon is more pronounced in countries where investors are better protected, and is less pronounced or even reversed in countries where investors are less protected.