The chloroplast NAD(P)H dehydrogenase (NDH) complex functions in PSI cyclic and chlororespiratory electron transport in higher plants. Eleven plastid-encoded and three nuclear-encoded subunits have been identified so far, but the entire subunit composition, especially of the putative electron donor-binding module, is unclear. We isolated Arabidopsis thaliana crr23 (chlororespiratory reduction) mutants lacking NDH activity according to the absence of a transient increase in Chl fluorescence after actinic light illumination. Although CRR23 shows similarity to the NdhL subunit of cyanobacterial NDH-1, it has three transmembrane domains rather than the two in cyanobacterial NdhL. Unlike cyanobacterial NdhL, CRR23 is essential for stabilizing the NDH complex, which in turn is required for the accumulation of CRR23. Furthermore, CRR23 and NdhH, a subunit of chloroplast NDH, co-localized in blue-native gel. All the results indicate that CRR23 is an ortholog of cyanobacterial ndhL in Arabidopsis, despite its diversity of structure and function.