Background: Gestational diabetes mellitus (GDM) is a current health problem that affects pregnant women and fetuses. Addressing GDM offers a window of opportunity for the prevention of DM in Saudi Arabia. Here, we aimed to assess the current level of knowledge regarding GDM among prenatal women attending primary healthcare centers (PHCs) in the city of Al-Khobar. We also aimed to increase the awareness of GDM and to determine the significance of this study within the Saudi Arabian context. Methods: After obtaining informed consent from the study subjects, a pre-validated, structured questionnaire was used to assess GDM knowledge among prenatal women attending PHCs in the city of Al-Khobar. Results: We included 471 prenatal women in the study (mean age: 29 years). Overall, 36.5% of the women had good knowledge of GDM, while 50.5% had fair knowledge, and 13% had poor knowledge. Most women (> 99%) were literate and identified themselves as housewives (75.4%). The knowledge score concerning how GDM is diagnosed was 45.3%. Approximately 41.4% women had undergone tests for GDM. Furthermore, 65.8% of the participants were aware of GDM treatment. However, only 39.5% were aware of the consequences of GDM. A major source of GDM knowledge was reported to be from friends and relatives. Conclusion: Although knowledge of GDM as a disease in Al-Khobar community was average, the depth of this knowledge was poor. Thus, doctors and healthcare providers must play a greater role in improving health education among prenatal women. Furthermore, suitable intervention programs should urgently be undertaken by the Ministry of Health to improve GDM knowledge among this population.