Genome size (C value) is a fundamental characteristic of every species and is very important for the progress of cytogenetic, genomic, and phylogenic studies. However, information on the C value of phylum Brachiopoda is scarce. In this study, we collected 8 brachiopod species from Japan, including 5 from class Articulata, and determined their C values by flow cytometry. The mean C values for these 8 species-namely, Lingula anatina, Lingula reevii, Discradisca stella, Terebratulina crossei, Laqueus rubellus, Laqueus blanfordi, Terebratalia coreanica, and Platidia japonica—were 0.41 pg, 0.41 pg, 0.46 pg, 0.33 pg, 0.44 pg, 0.42 pg, 0.37 pg, and 0.31 pg, respectively. Although the C values were examined across the various taxa within Brachiopoda, we detected very little variation (approximately 1.5-fold, from 0.31 pg in P. japonica to 0.46 pg in D. stella). This low C value variation suggests that the occurrences of evolutionary events such as changing the number of transposable elements and intron size, polyploidy, and DNA loss—all of which could cause a change in genome size—are fewer than in other taxa of the animal kingdom.