Small proteins are an under investigated class of gene products in all domains of life. Here we describe the role of NsiR6/NblD, a cysteine-rich 66 amino acid small protein in the acclimation response of cyanobacteria to nitrogen starvation. Phycobilisomes, the macromolecular pigment-protein complexes for photosynthetic light harvesting, are rapidly degraded upon nitrogen depletion. Deletion of nblD in Synechocystis sp. strain PCC 6803 prevents this degradation, indicated by the non-bleaching (nbl) phenotype. Complementation by a plasmid-localized gene copy fully restored the phenotype of the wild type, while overexpression of NblD under nitrogen-replete conditions did not lead to any phenotypical effect, different from the unrelated proteolysis adaptors NblA1 and NblA2, which can trigger phycobilisome degradation ectopically. However, transcriptome analysis revealed that nitrogen starvation induced nblA1/2 transcription in the ΔnblD strain, which excluded the possibility that the nbl phenotype was due to a possible NblD function as transcriptional co-regulator. In contrast, fractionation experiments indicated the presence of NblD in the phycobilisome fraction. Pull-down experiments with NblD fused to a triple FLAG tag identified the α and β phycocyanin subunits as the only two co-purifying proteins. Homologs of NblD exist in all cyanobacteria that use phycobilisomes but not in the genera Prochlorococcus and Acaryochloris which use alternative light-harvesting mechanisms. These data suggest that NblD plays a crucial role in the coordinated dismantling of phycobilisomes when nitrogen becomes limiting.