Significance In Arabidopsis , phytochrome B (phyB) perceives light and temperature signals to regulate various fundamental morphogenic processes in plants through its interconversion between its active Pfr and inactive Pr states. On photoconversion from Pr to Pfr, phyB forms subnuclear foci called photobodies, whose compositions and molecular functions are unclear. We show here that the phyB-interacting protein PCH1 is a structural component of phyB photobodies that protects Pfr from thermal reversion back to Pr, thereby helping to maintain phyB signaling. Loss of PCH1 compromises photobody formation, which disrupts a number of downstream events, including photomorphogenesis and thermal perception and signaling into the circadian clock. These results demonstrate that the formation of PCH1-dependent phyB photobodies is an essential step connecting light and temperature to controls on plant morphogenesis.