Thioredoxins (TRX) are ubiquitous proteins regulating the redox state of numerous proteins, in particular in chloroplasts. These small proteins have a particular fold and a CxxC motif comprising two active-site cysteines. A predicted chloroplastic protein possessing these characteristics has been automatically annotated as TRX-like but its function is unknown and its predicted fold is more related to thiol peroxidases (i.e. AhpC_TSA2-like domain). It is present in microalgae and land plants including trees and we referred it to as TRX-like3. The recombinant TRX-like3 proteins from Arabidopsis thaliana and Chlamydomonas reinhardtii have been expressed in Escherichia coli and purified. The structure of AtTRX-like3 has been solved. Like regular TRXs, it is a monomer and the CxxC motif present in the first -helix is surface exposed. However, AtTRX-like3 possesses an elongated N-terminal region and an insertion between the β3 and β4 strands compared to conventional TRXs. While both TRX-like3 are oxidized by H2O2, no thiol peroxidase activity has been detected. A reductase activity was not detected either, using insulin as a substrate. Since the biochemical analyses do not give information about a possible function as a reductase, Chlamydomonas mutants have been selected. Preliminary results indicate that Trx-like3 mutants exhibit a growth retardation on a minimal medium under light which was not visible when grown photo-autotrophically on rich-media. This opens the way for a deeper functional analysis.