The fauna of Eumolpinae of New Caledonia has been under intense scrutiny in over the last decade and numerous species are being described on a regular basis. In this study, we revise in detail the group of Colaspis laboulbenei Montrouzier, 1861, currently in the genus Dematochroma Baly, 1864, using a combined approach with DNA-based phylogenies and the study of morphology and species distributions. Several relevant systematic conclusions result from this revision: (1) recognition of the monophyletic group around Dematochroma laboulbenei; (2) recognition of an important morphological character, namely the presence or absence of a dorsal groove on pygidium, separating two narrowly parapatric lineages in this group; (3) reversal of a long-held synonymy with D. laboulbenei and reinstatement of Thasycles cordiformis Chapuis, 1874 stat. rev.; (4) restitution of Thasycles Chapuis, 1874 stat. rev. based on phylogenetic and taxonomic arguments to delimit this group of species; (5) transfer of Dematochroma fusca Jolivet, Verma & Mille, 2007, D. laboulbenei and Dematochroma panieensis Jolivet et al., 2007 to Thasycles; and (6) description of seven new species, including Thasycles castaneus Gómez-Zurita n. sp., Thasycles compactus Gómez-Zurita n. sp., Thasycles grandis Gómez-Zurita n. sp., Thasycles magnus Gómez-Zurita n. sp., Thasycles puncticollis Gómez-Zurita n. sp., Thasycles tenuis Gómez-Zurita n. sp., and Thasycles variegatus Gómez-Zurita n. sp. A species identification key and a brief discussion about the systematics and potential factors affecting the diversification and distribution of Thasycles are provided. This work was possible thanks to several sources of funding, including Grant CGL2017-83324-P of the Spanish Ministry of Science and Innovation, with support from the European Regional Development Fund, and also to the European Commision Synthesys+ project (grant no. BE-TAF-81). Abstract Keywords 1. Introduction 2. Material and methods 2.1. Sampling 2.2. Acquisition of DNA sequence data 2.3. Phylogenetic analyses 2.4. Morphological analyses 3. Results and discussion 3.1. Phylogenetic relationships of the groups of interest 3.2. Restatement and limits of the genus Thasycles Chapuis, 1874 3.3. Redescription of Thasycles 3.4. Fixing the meaning of old names 3.5. Taxonomic treatment 3.6. Identification key to the species of Thasycles 3.7. Evolution of Thasycles and the classification of Eumolpinae Declaration of competing interest Acknowledgements Appendix A. Supplementary data References