Abstract This paper analyzes the relationship between GDP and the stock market over the long term, intending to understand the implications for Index Fund investments. A quantitative research method, using US (United States) GDP as an independent variable, and the S&P 500 index as a dependent variable, is employed. A population of 29 years, from 1990 to 2019, of data on US GDP and the S&P 500 from official US sources was used. Linear regression analysis with SPSS calculating techniques is performed to determine whether there is a relationship between GDP growth and the stock market (S&P 500). The results show a significant positive relationship between GDP growth and S&P 500 performance. β coefficient of the regression analysis of 0.911 shows a strong correlation between the GDP and the S&P 500. Our findings are also scientifically validated by the sig (P value) coefficient of 0.0000000000012. In addition, an R Square of 0.830 shows that our model explains all the variability of the response data around the mean at a level of 83%. The positive results of GDP and the stock market relationship, indicate considerable implications for Index Funds investments. Therefore, adding academic value to the practical financial implication aspects.