Diatoms, a group of prevalent marine algae, contribute significantly to global primary productivity. Their substantial biomass is linked to enhanced absorption of blue–green light underwater, facilitated by fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs), which exhibit oligomeric diversity across diatom species. Using mild clear native PAGE analysis of solubilized thylakoid membranes, we displayed monomeric, dimeric, trimeric, tetrameric, and pentameric FCPs in diatoms. Mass spectrometry analysis revealed that each oligomeric FCP has a specific protein composition, and together they constitute a large Lhcf family of FCP antennas. In addition, we resolved the structures of the Thalassiosira pseudonana FCP (Tp-FCP) homotrimer and the Chaetoceros gracilis FCP (Cg-FCP) pentamer by cryoelectron microscopy at 2.73-Å and 2.65-Å resolution, respectively. The distinct pigment compositions and organizations of various oligomeric FCPs affect their blue–green light-harvesting, excitation energy transfer pathways. Compared with dimeric and trimeric FCPs, the Cg-FCP tetramer and Cg-FCP pentamer exhibit stronger absorption by Chl c, redshifted and broader Chl a fluorescence emission, and more robust circular dichroism signals originating from Chl a–carotenoid dimers. These spectroscopic characteristics indicate that Chl a molecules in the Cg-FCP tetramer and Cg-FCP pentamer are more heterogeneous than in both dimers and the Tp-FCP trimer. The structural and spectroscopic insights provided by this study contribute to a better understanding of the mechanisms that empower diatoms to adapt to fluctuating light environments.