Chemical examination of an arctic actinomycete Streptomyces nitrosporeus resulted in the isolation of two new alkaloids named nitrosporeusines A (1) and B (2), an unprecedented skeleton containing benzenecarbothioc cyclopenta[c]pyrrole-1,3-dione. Their structures were determined through extensive spectroscopic analyses in association with X-ray single crystal diffraction. Both 1 and 2 exhibited inhibitory activities against the H1N1 virus in MDCK cells.