The present study aims at revealing the relationships between hydrogen concentration and phase structure, as well as microstructure modification in the beta-metastable beta-21S titanium alloy. The beta-bcc phase can accommodate a large number of interstitial atoms, and hydrogenation by means of molecular hydrogen gas was employed in the present work. The phase structure as well as the microstructure of this alloy was found to be strongly dependent on hydrogen concentration. At lower hydrogen concentration (H/M <= 0.300), the microstructure consisting of the single beta-phase revealed that the interstitially dissolved hydrogen atoms expanded the bcc lattice and inhibited the decomposition of the beta phase upon cooling. The introduction of hydrogen beyond H/M = 0.300 was found to generate a large amount of internal stresses in the microstructure inducing the formation of metastable phases alpha'' in the form of lamellae and omega in the form of nanoparticles. The generation of the nanosized omega-phase was presumed to relax the strain caused by the volume expansion (2.28%) from the hydrogen-containing beta phase to the alpha'' martensite.