The ketogenic diet (KD) is a popular weight management method. However, knowledge, perceptions, and use of KD have not been studied in the Saudi population. This cross-sectional study aims to assess the knowledge, perceptions, and use of the KD in Saudi Arabia, to compare medical and nonmedical students in their awareness of KD, and to determine factors associated with knowledge and perception of KD. A total of 1071 Saudis aged ≥ 18 years old were included. Participants were excluded if they were younger than 18, non-Saudi, pregnant, breastfeeding, had chronic illnesses, were using any medication, or were diagnosed with psychiatric conditions. Knowledge, perception, and use of KD were collected between 2021 and 2022 in Saudi Arabia using a prevalidated tool and its scoring protocol; higher scores indicated higher level of knowledge or perception. Among Saudi adults, 24% (n = 253) and 55% of them (n = 138/253) followed the KD for 1 month or less, respectively. The KD knowledge score was mean ± SD: 7.95 ± 3.97 and half of the participants (49.5%) had a low knowledge level. Students had a significantly lower knowledge score (mean ± SD: 7.69 ± 3.85) compared to nonstudents (mean ± SD: 8.68 ± 4.19). Students in medical majors had a higher knowledge score compared to nonmedical major students (P < .001). The KD perception score was mean ± SD: 28.74 ± 4.83 and 76% of the participants had moderate perception level. Males had a significantly higher perception score compared to females (P < .001). Students in medical majors had a lower perception score compared to nonmedical major students. Age and body mass index had a significantly weak positive correlation with knowledge and perception scores. Half of the sample had low knowledge and moderate level of perception about KD. Students with medical majors had higher knowledge but lower perception scores compared to nonmedical majors. Dieticians may measure the knowledge and perception of the KD diet in outpatient clinics to lose weight. Educational institutions may increase students’ knowledge and perception of the KD in all majors. Future experimental studies examining the efficacy of the KD are needed to provide recommendations of the KD as a strategy for weight loss.