The analysis of the impact of a 5-min massage session on the gastrocnemius contractile and passive mechanical properties in young adults - a randomized controlled trial. The effect of massage therapy on the muscle properties is unclear. New technologies make investigation more in-depth and reliable. This study examined the effects of a 5-min massage session on the muscle stiffness and tone reduction in healthy young adults. Participants aged between 19 and 24 were randomly assigned to an intervention group (IG) (n = 40) or a control group (CG) (n = 40). The IG participated in a 5-min massage session of the lateral and medial gastrocnemius muscle (LGCM and MGCM respectively). The muscle contractile properties were assessed by tensiomyography (TMG). It measured maximal displacement (Dm), delay time (Td) and contraction time (Tc). Oscillation frequency (F) and stiffness (S) were used to assess muscle passive mechanical properties. These measurements were collected by MytonPro. There were no differences between the groups in the pre-test measurements. The results of mixed-model ANOVA showed significant (p < 0.05) time × group interactions, with greater alteration in the IG compared to the CG in the LGCM for Dm (%diff = 22.5, ƞp2 = 0.41), Td (%diff = 3.63, ƞp2 = 0.02), Tc (%diff = 13.38, ƞp2 = 0.1), F (%diff = 5.55, ƞp2 = 0.07) and in the MGCM for Dm (%diff = −14.34, ƞp2 = 0.06), F (%diff = −7.53, ƞp2 = 0.12). No difference (p > 0.05) was noticed in other variables. A 5-min massage session impacts the contractile and passive mechanical properties of the LGCM and MGCM. The size and direction of changes may depend on the muscle type, its geometry as well as other factors.