Chemical analysis of the marine-derived Saccharopolyspora sp. PG10 led to the isolation of a novel macrolactam, cebulactam A₃ (1), along with four known congeners, cebulactams A₁ and A₂ (2 and 3) and shengliangmycins B and D (4 and 5). The structure of 1 was established by high-resolution mass spectrometry (HRMS) and comprehensive nuclear magnetic resonance (NMR) analyses, and its absolute configuration was determined using Mosher’s method. Genome analysis identified a putative biosynthetic gene cluster consistent with a hybrid polyketide pathway. Antimicrobial evaluation revealed that shengliangmycin B exhibited the strongest activity, whereas cebulactam analogs exhibited weaker effects. These findings expand the structural diversity of cebulactam-type macrolactams and provide insights into their stereochemical variation.