Influenza infection continues to be a hazard to the Saudi population, resulting in high death rates and illness prevalence; it also places a substantial financial burden on the government. The government takes several strategies and approaches through the Ministry of Health has shown great success in curbing the disease. Vaccination is considered the most appropriate control measure; unfortunately, most Saudi residents, particularly in the city of Riyadh, do not consider vaccination a safe health practice. As a result, many have not participated in the influenza vaccine immunisation programme. Therefore, this study aimed to assess the knowledge, attitudes, and practices (KAPs) of seasonal influenza and influenza vaccine immunisation among clients visiting primary healthcare centers in Riyadh, Saudi Arabia. Furthermore, the study investigated the relationship between participants' demographics and their KAPs regarding influenza vaccination. A cross-sectional, descriptive, correlational study was conducted among 611 individuals who visited four of Riyadh's primary healthcare centers: Alsylimania, Alwady, Alyasmin, and Alsahafah. A self-reported questionnaire was used to assess the KAPs of participants, with questions regarding seasonal influenza and influenza vaccine immunisation. The scores of participants' knowledge showed that the majority had good knowledge regarding seasonal flu (64.5%) and the flu vaccine (73.3%). Furthermore, only 52% of participants had a positive attitude score towards the seasonal influenza vaccination. Despite that, significant knowledge gaps and mistaken beliefs regarding certain aspects of influenza were noted in participants, resulting in negative attitudes and perceptions as well as a reduced likelihood of being vaccinated. In this study, 43.7% of participants (267 out of 611) had ever received a flu vaccine. Participants with a history of previous vaccination had a significantly higher level of knowledge and more positive attitudes, which resulted in increased vaccination coverage. Therefore, educational strategies to improve knowledge regarding influenza in Riyadh are recommended.