The single-item Healthy Diet Scale (HDS) was developed to estimate the percentage of daily diet that consumers regard as healthy. The aim of the presented studies was to demonstrate its validity and reliability. To evaluate its validity, in Study 1, N = 108 Dutch young adults completed a survey including the HDS (with picture examples) and the multiple-item Start the Conversation diet scale (STC). To examine the reliability of the HDS, a same-day test-retest assessment was conducted. In Study 2, an online survey among N = 154 Dutch students compared the HDS with and without picture examples. In Study 3, among N = 302 Turkish students, the HDS (without picture examples) was compared with the STC. In Study 4, among N = 37 Dutch and N = 83 English language students, the test-retest reliability of the HDS (without picture examples) was assessed for a longer time interval. Study 5, among N = 100 Dutch students, evaluated the correlation of the HDS (without picture examples) with various health outcomes. In Study 1, the analysis revealed a significant correlation between the HDS with picture examples and the STC. The intraclass correlation (ICC) between the test and retest assessment of 0.982 suggested excellent agreement between the two assessments. Study 2 revealed no significant difference in the HDS with and without example pictures. Study 3 revealed that the outcome of the HDS without picture examples correlated significantly with the STC. Study 4 revealed that the 11-day test-retest reliability of the HDS without picture examples was good to excellent. Study 5 revealed a significant association between the HDS without picture examples and immune fitness, sleep quality, insomnia, happiness and quality of life, but not with mood. Taken together, the HDS is a valid and reliable scale to estimate the percentage of daily diet that consumers perceive as healthy.