Soccer is the most popular sport globally, cherished by various societal groups. Soccer enthusiasts who support a match are commonly referred to as supporters. The supporter culture encompasses various subcultures, including the 'ultras' known for their hardcore and fanatic spirit. This fanatic spirit can lead to fanaticism, an excessive passion for something, and steadfastly holding onto one's beliefs. Fanaticism can have both positive and negative impacts. This study aims to describe the fanaticism of Persibas Banyumas soccer supporters, specifically the Knight Rebel, through the aspects of fanaticism. Sample selection employed purposive sampling with a total of four research participants. Data collection techniques used in this study included semistructured interviews and non-participant observation. The data analysis technique utilized was the phenomenological data analysis method using interpretative phenomenological analysis (IPA). The research results show that the fanaticism described within the Knight Rebel supporter group includes experiencing an innate connection, acting based on profound love, primordialism, solidarity, emotional feelings towards others, actions taken against others, labeling by others, the influence of others, and the psychological impact of literature. Other findings revealed factors leading to fanaticism, such as disagreements with previous groups, the desire to vent negative emotions, excessive love, and the influence of soccer broadcasts on television. Further findings on the characteristics of fanaticism in this group include viewing their traditions as correct, scolding those with opposing views, and possessing high loyalty.