Three new indolediketopiperazine alkaloids of the isoechinulin type, rubrumazines A-C (1-3), each possessing an oxygenated prenyl group either at C-7 (1 and 2) or at C-5 (3), along with 13 related analogues (4-16), were isolated and identified from a culture extract of Eurotium rubrum MA-150, a fungus obtained from mangrove-derived rhizospheric soil collected from the Andaman Sea coastline, Thailand. Their structures were established by detailed interpretation of NMR spectroscopic and mass spectrometry data analysis. The structure and absolute configuration of compound 1 were confirmed by X-ray crystallographic analysis, thus providing the first characterized crystal structure of an isoechinulin-type alkaloid. All isolated compounds were evaluated for brine shrimp lethality and antibacterial activity.