Assessing healthy diet literacy and eating behaviors is critical for identifying appropriate public health responses to the COVID-19 pandemic. We examined the psychometric properties of digital healthy diet literacy (DDL) and its association with eating behavior changes during the COVID-19 pandemic among nursing and medical students. We conducted a cross-sectional study from 7 April to 31 May 2020 at 10 public universities in Vietnam, in which 7616 undergraduate students aged 19–27 completed an online survey to assess socio-demographics, clinical parameters, health literacy (HL), DDL, and health-related behaviors. Four items of the DDL scale loaded on one component explained 71.32%, 67.12%, and 72.47% of the scale variances for the overall sample, nursing, and medical students, respectively. The DDL scale was found to have satisfactory item-scale convergent validity and criterion validity, high internal consistency reliability, and no floor or ceiling effect. Of all, 42.8% of students reported healthier eating behavior during the pandemic. A 10-index score increment of DDL was associated with 18%, 23%, and 17% increased likelihood of healthier eating behavior during the pandemic for the overall sample (OR, 1.18; 95%CI, 1.13, 1.24; p < 0.001), nursing students (OR, 1.23; 95%CI, 1.10, 1.35; p < 0.001), and medical students (OR, 1.17; 95%CI, 1.11, 1.24; p < 0.001), respectively. The DDL scale is a valid and reliable tool for the quick assessment of digital healthy diet literacy. Students with higher DDL scores had a higher likelihood of healthier eating behavior during the pandemic.