The dynamic shuttling of proteins between the nucleus and cytoplasm orchestrates vital functions in eukaryotes. Here, we unveil multifaceted functions of Arabidopsis Sin3-associated protein 18 kDa (SAP18) in regulating development and stress tolerance. As a crucial component of the Apoptosis- and Splicing-Associated Protein (ASAP) complex in the nucleus, SAP18 plays an integral role in the precise splicing of genes associated with leaf development, decoding its significance in post-transcriptional regulatory networks. Under heat shock conditions, SAP18 relocates to cytoplasmic stress granules and processing bodies, suggesting a novel thermoprotective role. This dual localization highlights SAP18 versatility in coordinating cellular responses, encompassing both nuclear splicing regulation and cytoplasmic stress granule dynamics. Our findings significantly expand the understanding of the intricate involvement of SAP18 in plant growth, and pave the way for future exploration of its diverse functions in thermotolerance mechanisms.