Significance: Remarkable chemical families are being recognized by studying diverse symbioses. We identified, through metagenomics, the first cyanobacterial trans -AT polyketide biosynthetic pathway in the Nostoc symbiont of the lichen Peltigera membranacea and showed its expression in natural thalli. An isotope-based technique designed for characterizing minute amounts of material confirmed predictions that its product, nosperin, is a distinct member of the pederin family of compounds that was previously thought exclusive to animal–bacteria associations. The unexpected discovery of nosperin in lichen expands the structural range and known distribution of this family of natural products and suggests a role associated with symbiosis.