Stress granules (SGs) are membraneless organelles that form in eukaryotic cells after stress exposure [1] (reviewed in [2–4]). Following translation inhibition, polysome disassembly releases 48S preinitiation complexes (PICs). mRNA, PICs, and other proteins coalesce in SG cores [1, 5–7]. SG cores recruit a dynamic shell, whose properties are dominated by weak interactions between proteins and RNAs [8–10]. The structure and assembly of SGs and how different components contribute to their formation are not fully understood. Using super-resolution and expansion microscopy, we find that the SG component UBAP2L [11, 12] and the core protein G3BP1 [5, 11–13] occupy different domains inside SGs. UBAP2L displays typical properties of a core protein, indicating that cores of different compositions coexist inside the same granule. Consistent with a role as a core protein, UBAP2L is required for SG assembly in several stress conditions. Our reverse genetic and cell biology experiments suggest that UBAP2L forms granules independent of G3BP1 and 2 but does not interfere with stress-induced translational inhibition. We propose a model in which UBAP2L is an essential SG nucleator that acts upstream of G3BP1 and 2 and facilitates G3BP1 core formation and SG assembly and growth.