Several populations of a partially submerged, robust, morphologically distinct Fossombronia were personally collected in central Mexico and several high elevation subpáramo communities in Venezuela. Comparative morphological studies of these populations with the type collections of F. peruviana, F. herzogii, F. ptychophylla, and F. australis, and a phylogenetic analysis of three chloroplast loci in 18 morphologically defined species of the genus were conducted to determine the taxonomic status of these populations. Both studies support the recognition of these populations as a new species of Fossombronia, which is named and described herein as Fossombronia delgadilloana. This new species resolves in a clade that is sister to F. peruviana, and is characterized by horizontal, planate, sub-longitudinally inserted leaves, widely dispersed antheridia, with basally expanded stalks, stipitate caulocalyces that lack lamellae, obovoid to ellipsoidal capsules, I-type inner capsule wall thickenings, and spores that are > 40µm, with reticulate distal wall ornamentation. Fossombronia herzogii is recognized as a new synonym of F. peruviana, and a new lectotype is designated for F. ptychophylla.