CARM1/PRMT4 (for COACTIVATOR-ASSOCIATED ARGININE METHYLTRANSFERASE1/PROTEIN ARGININE METHYLTRANSFERASE4) catalyzes asymmetric dimethylation on arginine (Arg), and its functions in gene regulation is understood only in animal systems. Here, we describe AtPRMT4a and AtPRMT4b as a pair of Arabidopsis (Arabidopsis thaliana) homologs of mammalian CARM1/PRMT4. Recombinant AtPRMT4a and AtPRMT4b could asymmetrically dimethylate histone H3 at Arg-2, Arg-17, Arg-26, and myelin basic protein in vitro. Both AtPRMT4a and AtPRMT4b exhibited nuclear as well as cytoplasmic distribution and were expressed ubiquitously in all tissues throughout development. Glutathione S-transferase pull-down assays revealed that AtPRMT4a and AtPRMT4b could form homodimers and heterodimers in vitro, and formation of the heterodimer was further confirmed by bimolecular fluorescence complementation. Simultaneous lesions in AtPRMT4a and AtPRMT4b genes led to delayed flowering, whereas single mutations in either AtPRMT4a or AtPRMT4b did not cause major developmental defects, indicating the redundancy of AtPRMT4a and AtPRMT4b. Genetic analysis also indicated that atprmt4a atprmt4b double mutants phenocopied autonomous pathway mutants. Finally, we found that asymmetric methylation at Arg-17 of histone H3 was greatly reduced in atprmt4a atprmt4b double mutants. Taken together, our results demonstrate that AtPRMT4a and AtPRMT4b are required for proper regulation of flowering time mainly through the FLOWERING LOCUS C-dependent pathway.