Stock selection is one of the most important decisions made by investors in the capital market, directly affecting investment returns and risk. Despite the importance of fundamental analysis, the lack of a coherent and localized framework for transforming financial information into comparable evaluation criteria remains a major challenge for investors. This study aims to develop and evaluate the Fundamental Investing System (FIS), a multi-factor fundamental scoring model designed to transform stock selection into a systematic and objective process. The FIS is based on 11 fundamental variables and ranks listed companies on a scale from 0 to 100. To evaluate its performance, all eligible companies listed on the Tehran Stock Exchange were scored in June 2023, and the top 30 companies were selected to construct an investment portfolio. The portfolio's performance was examined over the period from June 2023 to June 2026 using a buy-and-hold strategy and compared with the Tehran Stock Exchange Total Index, the U.S. dollar exchange rate, fixed-income funds, and the inflation rate. The results showed that the FIS-based portfolio achieved a cumulative return of 427%, outperforming the Total Index (133%), the U.S. dollar (234%), fixed-income funds (130%), and inflation (197%). Furthermore, a strong positive relationship was observed between the fundamental score and the three-year stock return (Pearson correlation coefficient = 0.8638; coefficient of determination = 0.746). Favorable values of Maximum Adverse Excursion (MAE) and Maximum Favorable Excursion (MFE) further confirmed the system's ability to predict long-term stock performance and identify stocks with attractive risk–return characteristics.