To support the hypothesis that new Streptomyces species produce unique metabolites, we isolated Streptomyces sp. Sp080513GE-23—a new species of Streptomyces identified via 16S rRNA gene homology search—from the marine sponge Haliclona sp. and explored its secondary metabolites. The strain was cultured at 27 °C, and metabolites were extracted from the fermentation broth via sequential partitioning with ethyl acetate (EtOAc) and n-butanol (n-BuOH), followed by purification using repeated reversed-phase chromatography. Two new modified indole-containing tetrapeptides, designated JBIR-34 (1) and JBIR-35 (2), were obtained. The planar structures of 1 (molecular formula: C21H25ClN4O7) and 2 (C20H23ClN4O7, the 5-desmethyl analogue of 1) were elucidated using 1D and 2D nuclear magnetic resonance (NMR) spectroscopy and high-resolution electrospray ionization mass spectrometry (HRESIMS). Both compounds feature a modified indole moiety (substituted with chlorine, methyl, and hydroxy groups), a trisubstituted oxazoline moiety, and L-serine (L-Ser) and D-alanine (D-Ala) residues. The absolute configurations of the amino acid residues—L-Ser (2′′′S), D-Ala (2′′R), and the R-configured oxazoline-derived R-methylserine—were determined via acid hydrolysis and Marfey's method using NR-(5-fluoro-2,4-dinitrophenyl)-L-alaninamide (FDAA) derivatives. Activity assays revealed that 1 and 2 exhibited weak 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity with half-maximal inhibitory concentrations (IC50) of 1.0 mM and 2.5 mM, respectively—significantly weaker than the positive control R-tocopherol (IC50 = 50 µM). However, they showed no cytotoxicity against cancer cell lines or antibacterial activity against Micrococcus luteus and Escherichia coli. This study confirms that new sponge-derived Streptomyces species can produce metabolites with unique skeletal structures, validating the idea that new species generate novel compounds and highlighting the marine sponge Haliclona sp. as a source of undiscovered microorganisms capable of producing new substances.