Diplazium and allied segregates (Allantodia, Callipteris, Monomelangium) represent highly diverse genera belonging to the lady-fern family Athyriaceae. Because of the morphological diversity and lack of molecular phylogenetic analyses of this group of ferns, generic circumscription and infrageneric relationships within it are poorly understood. In the present study, the phylogenetic relationships of these genera were investigated using a comprehensive taxonomic sampling including 89 species representing all formerly accepted segregates. For each species, we sampled over 6000 DNA nucleotides of up to seven plastid genomic regions: atpA, atpB, matK, rbcL, rps4, rps4-trnS IGS, and trnL intron plus trnL-trnF IGS. Phylogenetic analyses including maximum parsimony, maximum likelihood and Bayesian methods congruently resolved Allantodia, Callipteris and Monomelangium nested within Diplazium; therefore a large genus concept of Diplazium is accepted to keep this group of ferns monophyletic and to avoid paraphyletic or polyphyletic taxa. Four well-supported clades and eight robust subclades were found in the phylogenetic topology. Reconstruction of the evolutionary pattern of morphological characters, such as dissections of leaves, petiole/rachis scales, and shapes of sori, recovered some character combinations of systematic value for infrageneric classification. In light of morphological characters and our molecular phylogeny, a re-defined Diplazium and an infrageneric classification are proposed.