In this study we present the NMR structure of the metallothionein (MT) from the snail Littorina littorea (LlMT) when complexed to Cd2+. LlMT is capable of binding 9 Zn2+ or 9 Cd2+ ions. Sequence alignments with other snail MTs revealed that the protein is likely composed of three domains. The study revealed that the protein is built in three individual domains, each folding into a single well-defined three-metal cluster. The center α2 and C-terminal β domains are in a unique relative orientation. Two variants with longer and shorter linkers were investigated, revealing that specific inter-domain contacts only occurred with the wild-type linker. Moreover, a domain-swap mutant, in which the highly similar α1 and α2 domains were exchanged, was structurally almost identical. It is suggested that the expression of a three domain MT confers to Littorina littorea an evolutionary advantage in coping with Cd2+ stress and adverse environmental conditions.