DNA methylation is a conserved epigenetic marker in plants and animals. InArabidopsis, DNA methylation can be established through an RNA-directed DNA methylation (RdDM) pathway. By screening for suppressors ofros1, we identified STA1, a PRP6-like splicing factor, as a new RdDM regulator. Whole-genome bisulfite sequencing suggested that STA1 and the RdDM pathway share a large number of common targets in theArabidopsisgenome. Small RNA deep sequencing demonstrated that STA1 is predominantly involved in the accumulation of the siRNAs that depend on both Pol IV and Pol V. Moreover, thesta1mutation partially reduces the levels of Pol V-dependent RNA transcripts. Immunolocalization assay indicated that STA1 signals are exclusively present in the Cajal body and overlap with AGO4 in most nuclei. STA1 signals are also partially overlap with NRPE1. Localization of STA1 to AGO4 and NRPE1 signals is probably related to the function of STA1 in the RdDM pathway. Based on these results, we propose that STA1 acts downstream of siRNA biogenesis and facilitates the production of Pol V-dependent RNA transcripts in the RdDM pathway.