Background & Aims Heart failure is one of the important cardiovascular diseases, affecting 1% of people in their 50s and 10% of people in their 80s. People with heart failure, in addition to physical pain, often experience stress, anxiety, depression, and poor quality of life (QoL). Considering the importance of illness-related worries and spiritual intelligence (SI) in these people, this study aims to determine the relationship between SI and illness-related worries in people with heart failure in Mazandaran, Iran. Materials & Methods This is a descriptive-correlational study. Participants were 247 patients with heart failure admitted to Fatemeh Al-Zahra Hospital affiliated to Mazandaran University of Medical Sciences. To measure the variables, a demographic form, the spiritual intelligence scale of King & Decicco (2009), and the illness-related worries questionnaire. Descriptive statistics (No. [%], Mean±, SD) and inferential statistics (Pearson correlation test, independent t-test) were used to analyze the data in SPSS software, version 16. Significance level was set at 0. 05. Results The mean total scores of SI and illness-related worries were 61. 20 ±, 16. 10 and 86. 70 ±, 19. 97, respectively. Pearson correlation test results showed a negative significant relationship between illness-related worries and SI (r=-0. 608, P=0. 001). Cognitive independence domain of illness-related worries was strongly correlated with all SI domains, including critical existential thinking, personal meaning production, transcendental awareness, and conscious state expansion. The SI had a significant relationship with gender (P=0. 001), marital status (P=0. 014), economic status (P=0. 002), employment status (P=0. 018), education status (P=0. 001), insurance coverage (P=0. 001). The illness-related worries had a significant relationship with gender (P=0. 001), history of diabetes (P=0. 04), marital status (P=0. 001), educational level (P=0. 044), and insurance coverage (P=0. 01). Conclusion To reduce the illness-related worries of people with heart failure, their SI should be improved using related interventions such as SI training programs.