To address the on-going need for chemical novelty and the limited information on Planctomycetota secondary metabolism, we focused on exploring the recently isolated marine planctomycetal strain Alienimonas chondri LzC2T to uncover its potential production of novel compounds. This work contemplates the description of a large-scale cultivation study of strain LzC2T, followed by metabolite extraction and compound isolation using chromatographic approaches, which resulted in the isolation of a novel molecule designated as alichondrichlorin. Structural elucidation of this new molecule was accomplished by a combination of high-resolution mass spectrometry and nuclear magnetic resonance. The molecule was additionally screened for anti-proliferative bioactivity against human tumoral and non-tumoral cell lines. These cytotoxicity assays revealed a targeted effect of alichondrichlorin in the growth of tumoral cell lines, especially human breast adenocarcinoma MCF-7 cell line (EC50 = 4.06 μM) without effect on the human non-tumoral THLE-2 cell line (EC50 > 50 μM).