The circadian oscillator allows organisms to synchronize their cellular and physiological activities with diurnal environmental changes. In plants, the circadian clock is primarily composed of multiple transcriptional–translational feedback loops. Regulators of post-transcriptional events, such as precursor messenger RNAs (pre-mRNA) splicing factors, are also involved in controlling the pace of the clock. However, in most cases the underlying mechanisms remain unclear. We have previously identified XAP5 CIRCADIAN TIMEKEEPER (XCT) as an Arabidopsis thaliana circadian clock regulator with uncharacterized molecular functions. Here, we report that XCT physically interacts with components of the spliceosome, including members of the Nineteen Complex (NTC). PacBio Iso-Seq data show that xct mutants have transcriptome-wide pre-mRNA splicing defects, predominantly aberrant 3′ splice site selection. Expression of a genomic copy of XCT fully rescues those splicing defects, demonstrating that functional XCT is important for splicing. Dawn-expressed genes are significantly enriched among those aberrantly spliced in xct mutants, suggesting that the splicing activity of XCT may be circadian regulated. Furthermore, we show that loss-of-function mutations in PRP19A or PRP19B, 2 homologous core NTC components, suppress the short circadian period phenotype of xct-2. However, we do not see rescue of the splicing defects of core clock genes in prp19 xct mutants. Therefore, our results suggest that XCT may regulate splicing and the clock function through genetically separable pathways