AbstractCryptophyte plastids originated from a red algal ancestor through secondary endosymbiosis. Cryptophyte photosystem I (PSI) associates with transmembrane alloxanthin-chlorophylla/cproteins (ACPIs) as light-harvesting complexes (LHCs). Here, we report the structure of the photosynthetic PSI–ACPI supercomplex from the cryptophyteChroomonas placoideaat 2.7-Å resolution obtained by crygenic electron microscopy. Cryptophyte PSI–ACPI represents a unique PSI–LHCI intermediate in the evolution from red algal to diatom PSI–LHCI. The PSI–ACPI supercomplex is composed of a monomeric PSI core containing 14 subunits, 12 of which originated in red algae, 1 diatom PsaR homolog, and an additional peptide. The PSI core is surrounded by 14 ACPI subunits that form 2 antenna layers: an inner layer with 11 ACPIs surrounding the PSI core and an outer layer containing 3 ACPIs. A pigment-binding subunit that is not present in any other previously characterized PSI–LHCI complexes, ACPI-S, mediates the association and energy transfer between the outer and inner ACPIs. The extensive pigment network of PSI–ACPI ensures efficient light harvesting, energy transfer, and dissipation. Overall, the PSI–LHCI structure identified in this study provides a framework for delineating the mechanisms of energy transfer in cryptophyte PSI–LHCI and for understanding the evolution of photosynthesis in the red lineage, which occurred via secondary endosymbiosis.