Hawaii is home to four species in the fern genus Diplazium: Diplazium arnottii, D. esculentum, D. molokaiense, and D. sandwichianum. Three are endemic to the Hawaiian Islands, and one is introduced and naturalized (D. esculentum). They vary in frequency, from very abundant (D. sandwichianum) to rare and critically endangered (D. molokaiense). Prior to this work, the phylogenetic relationships of only D. esculentum had been estimated due to lack of information available about these species. In this study, we inferred phylogenetic relationships of the Hawaiian Diplazium based on six chloroplast regions – atpA, atpB, matK, rbcL, rps4+rps4–trnS IGS, and trnL intron+trnL–trnF IGS. We downloaded these plastid markers from GenBank for an additional 83 Diplazium species and two Athyrium species, and used the latter as outgroups. The resulting phylogeny inferred from combined data indicated that D. arnottii and D. sandwichianum are sister taxa, likely with origins in the Paleotropics. Morphologically, the two species can be distinguished by leaf dissection and lengths of sori. Diplazium molokaiense is sister to the Asian D. heterocarpum (among our sampled taxa). Diplazium molokaiense is distinguished from other species in the clade by its large sori and longer fronds. Our results reveal the phylogenetic placements of three Diplazium species and corroborate the placement of a fourth species.