© 2019 Federation of European Biochemical Societies Phytochrome B (phyB) is a plant photoreceptor protein that regulates various photomorphogenic responses to optimize plant growth and development. PhyB exists in two photoconvertible forms: a red light-absorbing (Pr) and a far-red light-absorbing (Pfr) form. Therefore, to understand the mechanism of phototransformation, the structural characterization of full-length phyB in these two forms is necessary. Here, we report the molecular structure of Arabidopsis thaliana phyB in Pr form and the molecular properties of the Pfr form determined by small-angle X-ray scattering coupled with size-exclusion chromatography. In solution, the Pr form associated as a dimer with a radius of gyration of 50 Å. The molecular shape was a crossed shape, in which the orientation of the photosensory modules differed from that in the crystal structure of dimeric photosensory module. PhyB exhibited structural reversibility in the Pfr-to-Pr phototransformation and thermal reversion from Pfr to Pr in the dark. In addition, Pfr only exhibited nonspecific association, which distinguished molecular properties of Pfr form from those of the inactive Pr form.