In treating chronic illnesses, such as sickle cell disease (SCD), outpatient care is essential; poor adherence in attending clinic appointments can lead to serious outcomes. SCD is highly prevalent in Saudi Arabia, and patients with SCD are advised to follow up with their treating physician in order to control this disease manifestation and to better forecast its complications. Studies evaluating missed appointments among patients with SCD are rare. Therefore, the current study aimed to use the health belief model’s modifying factors in order to evaluate the variables associated with poor adherence in attending appointments. A total of 381 participants with SCD from various regions in the Jazan Province, southwestern Saudi Arabia, were included. The survey instrument included socioeconomic determinants, factors associated with poor adherence in attending outpatient appointments, and solutions under the conceptual framework of the health belief model. A descriptive analysis was conducted and the factors that impacted adherence in attending the appointments were evaluated. In the current sample, respondents with SCD from 21 to 30 years represented 41%, which was followed by participants who were 11 to 20 years at 21.5%. In addition, about 60% of the participants were women. Further, approximately 62% of the patients admitted were missing one or more outpatient appointments in the previous year, which was significantly related to various factors, such as socioeconomic characteristics and patient residence. Forgetting the appointment was the main reason for skipping outpatient appointments for patients with SCD; as such, reminders appear to be a good solution for most participants. Our findings indicated that modifying components of the health belief model, including age, level of education, income, patients’ residence, and lacking cues to action (such as reminders) are important in explaining the reason for poor adherence in attending appointments. Thus, efforts are needed to address these factors and to ensure that SCD patients uphold their appointments. Future studies should examine the clinical, psychological, and epidemiological aspects that are linked with missed consultations.