Abstract: A novel amide alkaloid, bursatamide A (1), featuring an unprecedented propyl-hexahydronaphthalene carbon framework, was isolated from the infrequently studied sea hare Bursatella leachi, alongside a new 3-phenoxypropanenitrile alkaloid, bursatellin B (2), and twelve known compounds. The structures of 1 and 2 were elucidated through comprehensive spectroscopic data analyses, while their relative and absolute configurations (ACs) were established through total synthesis and a series of quantum chemical calculations, including calculated electronic circular dichroism (ECD) spectra, optical rotatory dispersion (ORD) methods, and DP4+ probability analyses. Bursatamide A (1) demonstrated inhibitory effects against the human pathogenic bacteria Listeria monocytogenes and Vibrio cholerae. Erythro-bursatellin B (21), a diastereoisomer of 2, exhibited notable antibacterial activity against the fish pathogenic bacterium Streptococcus parauberis FP KSP28, with an MIC₉₀ value of 0.0472 μg·mL⁻¹.