Species delimitation in poorly-known and diverse taxa is usually performed based on monolocus, DNA barcoding-like approaches, while multilocus data are often used to test alternative species hypotheses in well-studied groups. We combined both approaches to delimit species in the Xenuroturris / Iotyrris complex, a group of venomous marine gastropods from the Indo-Pacific. First, COI sequences were analyzed using three methods of species delimitation, ABGD, PTP and GMYC to propose primary species hypotheses (PSH). Second, RAD-seq data were also obtained and an IQ-tree phylogenetic tree produced. We tested the impact of the level of missing data on the robustness of the phylogenetic tree obtained with the RAD-seq data. Alternative species partitions revealed with the COI dataset were also tested using the RAD-seq data and the BFD method. The congruence between the species hypotheses proposed with the mitochondrial gene and the clades in the RAD-seq tree, together with the morphological variability of the shell and the radula and the distribution pattern, was used to turn the PSH into secondary species hypotheses (SSH). Allopatric PSH defined with the COI gene were interpreted to correspond to intraspecific structure. Most of the species are found sympatrically in the Philippines, and only one is confidently identified as a new species and described as Iotyrris conotaxis n. sp. The results obtained demonstrate the efficiency of the combined monolocus/multilocus approach to delimit species.