Hydrogen desorption associated with thermally and deformation-induced α′ and ε martensitic transformations was characterized by cryogenic thermal desorption spectroscopy (C-TDS) during cooling, and by in situ mass spectroscopy under tensile deformation. The cooling process of C-TDS leads to the underlying relationship between the hydrogen desorption and martensitic transformations. As experimental results, both the α′ and ε martensitic transformations influenced the induction of hydrogen desorption. The dominant factor responsible for hydrogen desorption during the α′ martensitic transformation corresponded to the local changes in hydrogen diffusivity, while the hydrogen desorption during the ε martensitic transformation resulted from the motions of hydrogen-decorated transformation dislocations. Subsequently, in situ mass spectroscopy was performed under tensile deformation on a metastable steel with γ→ε→α′ transformation. The experiments also revealed that deformation-induced martensitic transformation was accompanied by hydrogen desorption. The obtained results can be utilized to study a dynamic effect of the hydrogen distribution on the hydrogen embrittlement susceptibility of the transformation-induced plasticity (TRIP) steels.