Background: Dietitians play a critical role in preventing food–drug interactions (FDIs) and drug–nutrient interactions (DNIs); however, evidence regarding their knowledge, attitudes, and practices (KAP) in Saudi Arabia remains limited. Objective: The aim of this study was to assess dietitians’ KAP regarding FDIs and DNIs and examine their associations with socio-demographic and professional characteristics. Methods: A national cross-sectional study was conducted among 353 dietitians using a validated and modified questionnaire. Knowledge was assessed via 15 multiple-choice items (score range 0–15) and categorized as poor (0–5), moderate (6–10), or good (11–15). Attitudes were assessed using 8 Likert-scale statements (score range 8–40) and classified as negative (8–19), neutral (20–29), or positive (30–40). Practices were assessed via 6 frequency-scale items (score range 6–30) and categorized as poor (6–14), moderate (15–23), or good (24–30). Associations were analyzed using chi-square tests. Results: In total, 65.2% of participants demonstrated poor knowledge. Knowledge level was significantly associated with nationality (p = 0.011), educational qualification (p = 0.042), attendance at FDI/DNI training courses (p = 0.003), and inclusion of related topics during university education (p = 0.013). Higher knowledge levels were also associated with managing digestive diseases (p = 0.001), cardiovascular diseases (p = 0.020), and cancer (p = 0.031). Positive attitudes were reported by 77.6% of participants and were significantly associated with managing cardiovascular disease (p < 0.001) and obesity (p = 0.008). Good practices were observed in 36.3% of dietitians and were significantly associated with younger age (p = 0.024), more recent graduation (p = 0.006), fewer years of professional experience (p = 0.002), and managing obesity (p = 0.016). Knowledge was positively associated with practice (p < 0.001). Conclusions: Despite generally positive attitudes, substantial gaps in knowledge and practice regarding FDIs and DNIs exist among dietitians in Saudi Arabia. Strengthening academic curricula and continuing professional education is essential to enhance competency and improve patient safety.