This study aimed to evaluate the influence of El Niño and La Niña phase variability on autumn precipitation over western Iran based on the Niño 3.4 Index. Two datasets were employed: monthly precipitation records from the Khorramabad synoptic meteorological station and monthly Niño 3.4 index values obtained from National Oceanic and Atmospheric Administration (NOAA) for the 20-year period 2000–2019. Pearson’s correlation coefficient was applied to quantify the statistical relationship between regional precipitation and ENSO variability, while ENSO phases were classified using a fuzzy threshold criterion. The results revealed a direct teleconnection between autumn precipitation and the Niño 3.4 index, with the strongest and statistically significant relationship observed in October at the 95% confidence level. In contrast, the relationships in November and December were not statistically significant. Mean precipitation during October, November, and December was generally higher under El Niño conditions compared to La Niña phases. Precipitation variability in October and November was greater during El Niño events, whereas in December higher dispersion was associated with La Niña conditions. Over the 20-year study period, maximum precipitation in October and November predominantly occurred during El Niño phases, while in December peak precipitation was recorded during La Niña episodes. The highest mean precipitation was observed in December (88.13 mm) during El Niño conditions, whereas the maximum precipitation variability in December (62.52) occurred under La Niña influence. Overall, the findings highlight a seasonally differentiated ENSO signal in autumn precipitation over Khorramabad, with a stronger and more consistent influence detected in early autumn.