Background: Health Literacy (HL) is one of the main determinants of health and is crucial for the prevention of noncommunicable diseases, by influencing key health-related behaviours. The aim of the present study was to assess the role of HL and sociodemographic factors in predicting the adoption of two healthy behaviours—physical activity and fruits and vegetables consumption. Methods: This study was conducted on the Tuscan population subsample of the Italian Behavioral Risk Factor Surveillance System in 2017–2018. HL was assessed using the Italian version of the six-item European Health Literacy Survey Questionnaire (HLS-EU-Q6). Results: About 40% of the 7157 interviewees reported an inadequate or problematic HL level. Female sex, poor financial status, foreign nationality, and low education were associated with a problematic HL level, while an inadequate HL level was associated with being 50–69 years old, low education level, foreign nationality, poor financial status and unemployment or inactive status. Inadequate HL level was a strong predictor of both eating less than three portions of fruits/vegetables per day and not engaging in sufficient PA during leisure times. Conclusions: Our findings showed that an inadequate level of HL could negatively affect physical activity and diet, independently from the other sociodemographic conditions, confirming the role of HL as a relevant social determinant of health.