This paper provides a mechanistic understanding of hydrogen-microstructure-strain interactions in a finely-grained 25Cr-7Ni super duplex stainless steel subjected to dynamic tensile loading. Miniature-sized tensile specimens were hydrogen-charged for up to nine days, and the microstructure was imaged, in-situ, during mechanical tensile testing. Digital image correlation analysis of the recorded images revealed that the austenite phase underwent softening while the ferrite phase hardened due to uptaken hydrogen. Severe strain localization occurred due to dissolved hydrogen in the microstructure resulting in hydrogen-induced cracks. Mobile hydrogen atoms caused softening of the microstructure while trapped hydrogen reasoned hardening. The austenite's hydrogen absorption capacity is decisive for the susceptibility to hydrogen embrittlement.