A new potential light-harvesting protein, named Lhca5, was recently detected in higher plants. Due to the low amount of Lhca5 in thylakoid membranes, the isolation of a native Lhca5 pigment-protein complex has not been achieved so far. Therefore, we used in vitro reconstitution to analyze whether Lhca5 binds pigments and is actually an additional light-harvesting protein. By this approach, we could demonstrate that Lhca5 binds pigments in a unique stoichiometry. Analyses of pigment requirements for LHC formation by Lhca5 revealed that Chlorophyll b is the only indispensable pigment. Fluorescence measurements showed that ligated chlorophylls and carotenoids are arranged in a way that allows directed energy transfer within the LHC. Reconstitutions of Lhca5 together with other Lhca proteins resulted in the formation of heterodimers with Lhca1. This demonstrates that Lhca5 is indeed a protein belonging to the light-harvesting antenna of photosystem I. The properties of Lhca5 are compared with those of previously characterized Lhca proteins and the consequences of an additional Lhca protein for the composition of the light-harvesting antenna of Photosystem I are discussed in view of the recently published Photosystem I structure of pea.