Summary Thioredoxins (TRXs) are ubiquitous disulfide oxidoreductases structured according to a highly conserved fold. TRXs are involved in a myriad of different processes through a common chemical mechanism. Plant TRXs evolved into seven types with diverse subcellular localization and distinct protein target selectivity. Five TRX types coexist in the chloroplast, with yet scarcely described specificities. We solved the crystal structure of a chloroplastic z‐type TRX, revealing a conserved TRX fold with an original electrostatic surface potential surrounding the redox site. This recognition surface is distinct from all other known TRX types from plant and non‐plant sources and is exclusively conserved in plant z‐type TRXs. We show that this electronegative surface endows thioredoxin z (TRXz) with a capacity to activate the photosynthetic Calvin–Benson cycle enzyme phosphoribulokinase. The distinct electronegative surface of TRXz thereby extends the repertoire of TRX–target recognitions. Significance Statement The high‐resolution crystal structure of thioredoxin z (TRXz) from the model alga Chlamydomonas reinhardtii confirms that TRXz is generally structured as a canonical redox TRX but exposes a type‐specific electronegative surface around its active site. The ability of TRXz to interact with its target proteins is determined by this surface and reveals a compatibility with Calvin–Benson phosphoribulokinase that proved to be activated by TRXz in vitro. TRXz hence appears as a novel player in photosynthesis.