The Forster–Decker method is a series of chemical reactions that have the effect of mono-alkylating a primary amine (1), forming a secondary amine (6). The process occurs by way of transient formation of an imine (3) that undergoes the actual alkylation reaction.

Because the actual alkylation occurs on the imine, over-alkylation is not possible. Therefore, this method does not suffer from side-reactions such as formation of tertiary amines as a simple SN2-type process can.