The genus Diaporthe comprises close to 800 species, with around 2000 names attributed to it and its asexual morphs previously recognized in Phomopsis. Diaporthe species are common plant associates, including saprotrophs, pathogens, and endophytes affiliated with a diverse range of hosts worldwide. In this study, an unknown Diaporthe sp. was frequently isolated as an endophyte from healthy Picea mariana and Picea rubens needles in the Acadian forest of Eastern Canada. Morphological observations and the application of the genealogical concordance phylogenetic species recognition concept using four unlinked loci (internal transcribed spacer (ITS), DNA-lyase (Apn2), translation elongation factor 1-α (EF1-α), and beta-tubulin (TUB)) support the distinctiveness of this species, described here as Diaporthe maritima. Crude liquid culture extracts from this new species showed potent antifungal activity towards the biotrophic pathogen Microbotryum violaceum in a screening assay necessitating an investigation of its natural products. Three dihydropyrones, phomopsolides A (1), B (2), and C (3), and a stable alpha-pyrone (4), were characterized by mass spectrometry and spectroscopic techniques. All isolated metabolites individually demonstrated in vitro antifungal and antibiotic activity towards Bacillus subtilis. The Acadian forest has proven to be a rich source of biodiversity that has the potential to deliver environmentally sustainable pest management tools.