The presence of introns in gene-coding regions is one of the most mysterious evolutionary inventions in eukaryotic organisms. It has been proposed that, although sequences involved in intron recognition and splicing are mainly located in introns, exonic sequences also contribute to intron splicing. The smallest constitutively spliced exon known so far has 6 nucleotides, and the smallest alternatively spliced exon has 3 nucleotides. Here we report that the Anaphase Promoting Complex subunit 11 (APC11) gene in Arabidopsis thaliana carries a constitutive single-nucleotide exon. In vivo transcription and translation assays performed using APC11-Green Fluorescence Protein (GFP) fusion constructs revealed that intron splicing surrounding the single-nucleotide exon is effective in both Arabidopsis and rice. This discovery warrants attention to genome annotations in the future.