This paper examines the effects of shareholder investment horizons on insider trading. We find that insiders are less likely to trade on private information and the profitability of insider trades is lower when shareholder investment horizons are longer. We further examine two channels through which shareholders with longer investment horizons can impede insider trading: direct monitoring and better information environment. Consistent with the direct monitoring channel, we show that insiders in firms with longer shareholder investment horizons are more likely to shift trades from the month right before earnings announcements to the month right after earnings announcements. Moreover, the impact of investment horizons are stronger in firms with higher ex ante litigation risk, with lower corporate governance quality, and that are not targets of hedge fund activists. Consistent with the information environment channel, we show that longer shareholder investment horizons increase the frequencies of information disclosure and insiders in firms with longer shareholder investment horizons are more likely to trade in an isolated manner rather than in sequences.