The current article introduces and describes Haslea berepwarisp. nov., a new species of diatom discovered in the vicinity of Boulouparis, New Caledonia. Under light microscopy, H. berepwarisp. nov. strongly resembles Haslea pseudostrearia, but preliminary molecular barcoding conducted using partial 18S and rbcL genes suggested that it was a distinct species. This was confirmed first by scanning electron microscopy which showed the differences in stria densities between both species. A short-reads genome-skimming protocol applied on H. berepwarisp. nov. led us to obtain its complete mitochondrial and plastid genomes. The mitogenome is 36,572 bp in length and as already observed among other species of Haslea spp., the nad6 and nad2 genes are fused within a single open-reading frame. The plastome is 131,897 bp length, and unlike the mitogenome, it is not colinear with those of H. pseudostrearia. The results derived from the sequencing of the plastome allowed to perform a 123-gene multigene maximum likelihood phylogeny that associates H. berepwarisp. nov. to H. pseudostrearia with maximum support at the nodes but also strictly distinguishes them, suggesting a greater genetic distance between these species than what has been previously observed between other marennine-producing species.