ABSTRACT 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 LzC2 T to uncover its potential production of novel compounds. This work contemplates the description of a large‐scale cultivation study of strain LzC2 T , 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 (EC 50 = 4.06 μM) without effect on the human non‐tumoral THLE‐2 cell line (EC 50 > 50 μM).