This study examines the effects of the Central Bank of the Republic of Turkey's (CBRT) policy changes in interest rates on the BIST Banking Index. Interest rates are of critical importance for the banking sector and areAa factor that can affect the behavior of investors and, thus, AstockAmarket indices. Between 23 September 2021 and 22 June 2023, Athe CBRT continuously lowered or kept interest rates low and decided to increase interest rates after this date. The study uses the event study method based on these two different policy change dates and focuses on measuring the effects of these changes on the BIST Bank Index. In this study using the event study method, the 120-day forecast window is preferred, while the (-5, +5) event window is used for abnormal and cumulative returns. The significance of the results is determined using a t-test. The results show that interest rate cut policies do not have a significant effect on the bank index, but interest rate hikes result in negative abnormal returns. The study contributes to the literature by analyzing the effects of these policy changes on the Borsa Istanbul Banking Index.