Adequate dietary protein intake is important for the maintenance of fat-free mass (FFM) and muscle strength, but optimal requirements remain unknown. Our aim in the current study was to explore the associations of protein intake with FFM and grip strength. We used baseline data from the UK Biobank (a study of 146,816 participants aged 40-69 years with data collected across the United Kingdom in 2007-2010) to examine the associations of protein intake with FFM and grip strength. Protein intake was positively associated with FFM (men: 5.1% (95% confidence interval (CI): 5.0, 5.2); women: 7.7% (95% CI: 7.7, 7.8)) and grip strength (men: 0.076 kg/kg (95% CI: 0.074, 0.078); women: 0.074 kg/kg (95% CI: 0.073, 0.076)) per 0.5-g/kg/day (grams per kg of body mass per day) increment in protein intake. FFM and grip strength were higher with higher intakes across the full range of intakes (i.e., highest in persons who reported consuming ≥2.00 g/kg/day) independently of sociodemographic factors, other dietary measures, physical activity, and comorbidity. FFM and grip strength were lower with age, but this association did not differ by category of protein intake (P > 0.05). The current recommendation for all adults (ages 40-69 years) to maintain a protein intake of 0.8 g/kg/day may need to be increased to optimize FFM and grip strength.