Melphalan flufenamide (melflufen, Pepaxto®) is a peptide conjugated alkylating drug developed by Oncopeptides for the treatment of multiple myeloma (MM) and amyloid light-chain amyloidosis. It is an ethyl ester of a lipophilic dipeptide consisting of melphalan and para-fluoro-L-phenylalanine. Due to its lipophilicity, melphalan flufenamide is rapidly transported across the cell membrane and almost immediately hydrolyzed by aminopeptidases in the cytoplasm to yield more hydrophilic alkylating molecules, such as melphalan and desethyl-melflufen. Like other nitrogen mustard drugs, melphalan flufenamide exerts antitumor activity through DNA crosslinking. In February 2021, melphalan flufenamide, in combination with dexamethasone, received its first approval in the USA for the treatment of adults with relapsed or refractory (r/r) MM who have received at least four prior lines of therapy and whose disease is refractory to at least one proteasome inhibitor (PI), one immunomodulatory agent, and one CD38-directed monoclonal antibody. A multinational clinical study of melphalan flufenamide in amyloid light-chain amyloidosis is underway across several countries, and preclinical studies for various haematological and solid cancers are underway. This article summarizes the milestones in the development of melphalan flufenamide leading to this first approval.