Purpose: To investigate the direct and indirect effects of sociodemographic/health factors on diet quality through practical nutrition knowledge (PNK) about how to compose a balanced meal. Design: A cross-sectional study using data from an online survey of the 10 000 Steps cohort (data collected November-December 2016). Setting: Australia. Participants: Adults (n = 8161). Response rate was 16.7%. Measures: Self-reported lifestyle, health, and sociodemographic characteristics, including diet quality and PNK. Analysis: The PROCESS macro for SPSS was used to conduct the mediation analyses. Results: Better diet quality was associated with being female, older, more highly educated, and having a lower body mass index. Mediation analysis showed that PNK significantly mediated the associations between sex (a*b = 0.54, 95% confidence interval CI = 0.39-0.70) and education (vocational education: a*b = 0.22, 95% CI = 0.12-0.35, university: a*b = 0.48, 95% CI = 0.35-0.64), and diet quality. Practical nutrition knowledge suppressed the association between age and diet quality (a*b = −0.03, 95% CI = −0.04 to −0.03). Conclusion: Variations in diet quality between sociodemographic groups were partially explained by differences in PNK, suggesting that focusing public health efforts on increasing this specific knowledge type might be promising.