The effects of hydrogen on microstructure evolution and superplastic behavior of a new near alpha high-temperature Ti-55 titanium alloy were studied. The results indicate gamma hydride with fct structure exists in the specimens when the hydrogen content is even up to 0.7 wt.%. The start temperature of alpha+beta/beta phase transition decreases with increasing hydrogen concentration. The optimum superplastic temperature of Ti-55 alloy with 0.1 wt.%H is reduced by about 125 degrees C. Hydrogenation can significantly reduce the flow stress. The elongation of the hydrogenated alloy is more sensitive to deformation temperature and strain rate during superplastic tension. The optimum superplastic temperature decrease is resulted from the comprehensive effects of lower dislocation density, alpha+beta/beta transus temperature reduction, dynamic recovery and recrystallization, twinning induced plasticity and more deformable beta phase after hydrogenation. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.