This study examines whether stock price fragility—the risk of price volatility arising from non-fundamental demand shifts, such as investor liquidity shocks—affects investment efficiency in real-economy firms. Our research indicates that firms facing higher price fragility exhibit significantly weaker investment performance. Price fragility amplifies noise trading and worsens information asymmetry, undermining the capacity of prices to convey reliable signals for managerial decisions. Further analysis reveals that financing constraints exacerbate the impact of stock price fragility on under-investment, whereas managerial overconfidence plays an asymmetric moderating role, mitigating under-investment while amplifying over-investment. Evidence from a natural experiment–based identification strategy supports a causal interpretation of our results and alleviates endogeneity concerns.