Background Lucidotini is a diverse tribe of lampyrine fireflies present throughout the New World, Europe, and Asia. Most of the over 30 genera have overlapping diagnoses, largely due to a lack of revisionary and phylogenetic studies. Widespread convergence in sensory morphology, traditionally used in genus-level diagnoses, further compounds the taxonomic issues surrounding the Lucidotini. Recent work has cast light on the value of terminalia and genitalic traits for Lucidotini taxonomy and called for a more thorough screening of morphological characters. Of special interest are basal outgrowths of the phallus ( i.e ., ventrobasal processes)—currently only known in Alychnus Kirsch and Photinus Laporte–that can be quite informative at the species level, but its variation within Lucidotini remains poorly studied. Most Lucidotini species remain only superficially described, while internal characters—including those of terminalia and genitalia—which could inform species identification and phylogenetic relatedness, remain unknown. Upon studying eight Lucidotini species superficially looking like Photinus and Photinoides McDermott—all of which bearing long ventrobasal processes–we raised the hypothesis that they belonged to a genus yet to be recognized. Methods Here, we analyzed 97 morphological characters of 32 lampyrid species spanning 17 of 30 Lucidotini genera under Bayesian Inference. Results We found evidence for the recognition and description of Saguassu gen. nov. to include seven new species ( Saguassu acutum sp. nov., Saguassu grossii sp. nov., Saguassu manauara sp. nov., Saguassu rebellum sp nov., Saguassu roura sp. nov., Saguassu serratum sp. nov. and Saguassu sinuosum sp. nov. ), in addition to Photinus dissidens Olivier ((transferred herein, thus generating Saguassu dissidens comb. nov. ), for which we also designate a lectotype and two paralectotypes). This previously neglected lineage of Lucidotini spans four South American biomes: Amazon, Atlantic Rainforest, Cerrado, and Pampa. Interestingly, Saguassu species span a gradient of morphologies related to signaling: from Lampyris -style ventrally bulging eyes, tiny antennae and no lanterns; intermediate eyes and antennae, with complete lanterns as in Photinus ; to small eyes and long antennae and small lanterns as in many Lucidota Laporte. Saguassu gen. nov. was consistently found closely related to the three other Lucidotini taxa with ventrobasal processes ( i.e ., Alychnus , Photinoides , and Photinus ). We provide an occurrence map of and a dichotomous key to Saguassu species, thoroughly compare this genus with co-occurring Lucidotini genera, and suggest steps towards a revision of the Lucidotini tribe.