Background: Preconception care (PCC) can contribute to reducing maternal and childhood mortality and morbidity, but the association is inadequately studied in Ethiopia, particularly in relation to the six Health Belief Model constructs. Therefore, this study aimed to determine the level of preconception care knowledge and associated factors among pregnant mothers. Methods: A hospital-based cross-sectional study was conducted at Nigist Eleni Mohammed Memorial Comprehensive Specialized Hospital from May to August 2022. A total of 332 pregnant women were selected through a systematic random sampling method. Data were collected using a structured, interviewer-administered, and pretested questionnaire. The data was entered into EpiData and exported into SPSS for analysis. Bivariate and multivariate analyses identified factors influencing preconception care knowledge (p < 0.05, 95% CI). Results: From 332 participants, less than one-half, 158 (47.6%, 95% CI: 42.3, 53.0%), participants had good knowledge of PCC. Knowledge scores varied across the Health Belief Model constructs, with participants showing the highest understanding of perceived benefits (53.3%) and self-efficacy (56%), and lower awareness for perceived susceptibility (40%) and cues to action (36.6%). Participants with college and above educational level (AOR = 4.17; 95% CI: 1.56, 11.18), previous cesarean delivery (AOR = 3.31; 95% CI: 1.29, 8.46), and husband support (AOR = 2.54; 95% CI: 1.12, 5.81) were factors that were significantly related to preconception care knowledge. Conclusions: Less than half of the participants had good knowledge of preconception care, revealing a significant awareness gap among women. These findings emphasize the need to enhance women’s education, involve men in reproductive health, and for healthcare providers use cesarean delivery encounters as key opportunities to educate women about PCC. In addition, health facility readiness should be improved to enhance PCC awareness.