On Sept. 17, the U.S. Federal Reserve cut the federal funds rate (overnight interest rate) for the first time in 2025. Here's what it could mean for the S&P 500 (^GSPC 0.34%) stock market index. According to the SEP, policymakers already favor an interest rate cut in October, followed by another in December. Wall Street appears to agree, because the CME Group's FedWatch tool places the odds of an October interest rate cut at 89%. However, the long-term direction of the S&P 500 is ultimately determined by corporate earnings rather than interest rates.