PurposeThe purpose of this paper is to investigate the association between aggressive tax planning and stock price synchronicity.Design/methodology/approachEmploying the special institutional background of China, this study constructs tax aggressiveness and stock price synchronicity measures for a large sample of Chinese stocks spanning the period 2003–2015. The authors employ OLS regression as the baseline methodology, and a fixed effect model, the Fama–Macbeth method and GMM as sensitivity checks. Matched samples and difference-in-difference analyses are used to control for endogeneity.FindingsThe authors find a significant and positive association between aggressive tax planning and stock price synchronicity. Because material information about risky tax transactions tends to be hidden in various tax accruals accounts, aggressive tax strategies make financial statements less transparent, thereby, increasing information asymmetry and decreasing stock price informativeness. The authors also find that the firms engaging in aggressive tax planning exhibit relatively high corporate opacity. In addition, the authors find that improvements in the tax enforcement regime, ownership status and high-quality auditors all constrain the adverse effects of tax aggressiveness.Practical implicationsThis study has important practical implications for China’s regulators, who are striving to reduce the tax burden of enterprises. It also helps investors to consider investment decisions more appropriately from a taxation perspective.Originality/valueFirst, this paper contributes to the stock price efficiency literature by identifying the effect of a hitherto unexamined factor, namely, firm-level aggressive tax planning, on the efficiency of stock prices. Second, this study provides further empirical evidence to support the agency view of tax aggressiveness, and the informational interpretation of stock price synchronicity. Third, this study helps us better understand the effects of firm-level tax policy on firm-specific information capitalization in an environment where overall country-level investor protection is relatively weak.