SignificanceDiatoms are mostly marine algae responsible for up to 20% of global carbon dioxide fixation. To overcome the slow and nonspecific properties of the CO2-fixing enzyme Rubisco, it is sequestered in a subcellular compartment called the pyrenoid, allowing CO2gas to be concentrated there. We discovered the repeat protein PYCO1, which can phase separate to form dense protein droplets in the test tube. The droplets specifically bind large quantities of diatom Rubisco resulting in much larger and less dynamic droplets. This phenomenon is caused by repeating elements on PYCO1 that oligomerize and bind to the Rubisco small subunits. Pyrenoids are ubiquitous and derived by convergent evolution. Understanding their formation provides a framework to introduce them into plants.