Portfolio optimization has become an essential investment strategy for achieving an effective balance between expected returns and investment risk in dynamic capital markets. This study investigates the construction of an optimal investment portfolio using Sharpe's Single Index Model with reference to twenty large-cap companies listed on the Bombay Stock Exchange (BSE). The research is based on secondary data collected from the Bloomberg Professional Database covering the period from January 2022 to December 2025. Financial indicators including mean return, standard deviation, beta coefficient, Excess Return-to-Beta (ERB) ratio, and cutoff rate (Ci) were computed to evaluate the investment efficiency of the selected securities. The findings indicate that Bajaj Finance Ltd., Tata Motors Ltd., and State Bank of India generated relatively higher average returns accompanied by higher systematic risk, whereas Nestlé India Ltd., Hindustan Unilever Ltd., and ITC Ltd. exhibited lower volatility and stable performance. The ERB analysis identified Mahindra & Mahindra Ltd., Bharti Airtel Ltd., Reliance Industries Ltd., and Bajaj Finance Ltd. as the most efficient securities for portfolio inclusion. The calculated cutoff rate facilitated the selection of stocks capable of maximizing risk-adjusted returns while maintaining portfolio diversification. The study concludes that the application of Sharpe's Single Index Model provides a practical and reliable framework for constructing efficient portfolios and supports informed investment decisions in the contemporary Indian equity market.....