Background <p>The MyotonPro is a widely used, safe, and non-invasive digital device for assessing muscle biomechanical and viscoelastic properties. This study aimed to determine the intra- and inter-session reliability of the MyotonPro in measuring the biomechanical and viscoelastic properties of the knee extensor and flexor muscles.</p> Methods <p>Eighty-four young male recreational athletes, with a mean age of 21.42 ± 1.37&#xa0;years, participated in a four-stage repeated-measure study. Both sessions followed the same protocol: rectus femoris (RF) and biceps femoris (BF) measurements were taken twice per side and averaged, with a 30-min interval between sets. Two different intraclass correlation coefficient (ICC) models were used: ICC_3,1 (two-way mixed effects, absolute agreement) for intra-session reliability and ICC_2,1 (two-way random effects, absolute agreement) for inter-session reliability. Standard error of measurement (SEM) was calculated using SEM = SD_pooled x √(1-ICC). The minimal detectable change (MDC₉₅) was calculated using the formula MDC₉₅ = 1.96 × √2 × SEM. Bland–Altman plots were generated to visually represent the agreement between the two sessions, complementing the ICC analysis.</p> Results <p>The MyotonPro demonstrated high to excellent intra-session reliability across all measured properties (tone, stiffness, elasticity, relaxation time, and creep) for both the RF (ICCs ≥ .95, ranging from .95 to .99; SEM: 0.01 to 6.14; MDC₉₅: 0.04 to 17.03) and the BF (ICCs ≥ .97, ranging from .97 to .99; SEM: 0.02 to 5.27; MDC₉₅: 0.05 to 14.62). The MyotonPro demonstrated excellent inter-session reliability across all measured properties for both the RF (ICCs ≥ .96, ranging from .96 to .99; SEM: 0.02 to 4.38; MDC₉₅: 0.05 to 12.14) and the BF (ICCs ≥ .94, ranging from .94 to .99; SEM: 0.02 to 5.17; MDC₉₅: 0.07 to 14.54). The Bland–Altman analysis confirmed near agreement for all measured properties across sessions 1 and 2 for both the RF and BF. The limits of agreement encompassed most data points, and the overall systematic mean bias was negligible, remaining close to zero.</p> Conclusions <p>This study establishes the MyotonPro as a highly reliable instrument for assessing muscle properties, evidenced by excellent intra-session and inter-session consistency across all measured biomechanical and viscoelastic properties. The negligible systematic bias indicates high temporal measurement stability, supporting the potential utility of myotonometry for monitoring changes in muscle mechanical properties in longitudinal assessments.</p>