Calcium (Ca2+) and redox signaling enable cells to quickly adapt to changing environments. The signaling protein calredoxin (CRX) from the green alga Chlamydomonas reinhardtii is a chloroplast-resident thioredoxin having Ca2+-dependent activity and harboring a unique combination of an EF-hand domain connected to a typical thioredoxin-fold. Using small-angle X-ray scattering (SAXS), FRET, and NMR techniques, we found that Ca2+-binding not only induces a conformational change in the EF-hand domain, but also in the thioredoxin domain, translating into the onset of thioredoxin redox activity. Functional analyses of CRX with genetically altered EF-hands revealed that EF-hand 4 is important for mediating the communication between the two domains. Moreover, we crystallized a variant (C174S) of the CRX target protein peroxiredoxin 1 (PRX1) at 2.4 Å resolution, modeled the interaction complex of the two proteins, and analyzed it by cross-linking and MS analyses, revealing that the interaction interface is located close to the active sites of both proteins. Our findings shed light on the Ca2+ binding-induced changes in CRX structure in solution at the level of the overall protein and individual domains and residues.