Aims The aim of this study was to utilize a cryptic biosynthetic gene cluster (BGC) of a marine proteobacterium Thalassomonas actiniarum for production of new lanthipeptides by heterologous expression system. Methods and Results Based on genome mining, a new BGC of class I lanthipeptide was found in the genome sequence of a marine proteobacterium T. actiniarum. Molecular cloning was performed to construct an expression vector derived from commercially available plasmid pET‐41a(+). Heterologous production of new lanthipeptides named thalassomonasins A and B was performed using the host Escherichia coli BL21(DE3) harbouring the expression vector. The structure of thalassomonasin A was determined by the interpretation of NMR and MS data. As a result, thalassomonasin A was determined to be a lanthipeptide with three units of lanthionine. The bridging pattern of the lanthionine rings in thalassomonasin A was determined by interpretation of NOESY data. The structure of thalassomonasin B was proposed by MS/MS experiment. Conclusions We succeeded in heterologous production of new class I lanthipeptides using a BGC of a marine proteobacterium T. actiniarum. Significance and Impact of the Study To the best of our knowledge, this is the first report of heterologous production of lanthipeptides derived from proteobacterial origin. There are many cryptic biosynthetic gene clusters (BCGs) of this class of lanthipeptides in proteobacterial genomes. This study may lead to the production of new lanthipeptides by utilizing the BCGs.