This study examines the impact of investor diversity on stock market volatility in the Casablanca Stock Exchange (CSE). The objective is to assess how different investor types—including institutional, individual, domestic, and foreign investors—affect market fluctuations. Using quarterly data from Q1 2008 to Q3 2024, we apply a GARCH (1, 1) model to extract conditional volatility measures, followed by a linear regression analysis and Granger causality tests to evaluate their influence on market volatility. The results indicate that foreign and institutional investor volatilities amplify market fluctuations, whereas domestic and individual investor volatilities exert a stabilizing effect. Additionally, Granger causality tests confirm that foreign, domestic, and individual investor volatilities significantly predict market volatility, while institutional investor volatility does not exhibit a causal relationship. These findings highlight the role of investor diversity in shaping market stability and suggest that balancing investor participation may help mitigate excessive volatility.