A diamine is an amine with two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term diamine refers mostly to primary diamines, as those are the most reactive.

In terms of quantities produced, 1,6-diaminohexane (a precursor to Nylon 6-6) is most important, followed by ethylenediamine. Vicinal diamines (1,2-diamines) are a structural motif in many biological compounds and are used as ligands in coordination chemistry. Many diamines are used as hardeners in the curing of epoxide resins. Putrescine and cadaverine are simple diamines that occur widely in nature.

Derivatives of ethylenediamine are prominent:

Xylylenediamines are classified as alkylamines since the amine is not directly attached to an aromatic ring.

Three phenylenediamines are known:

Various N-methylated derivatives of the phenylenediamines are known:

Examples with two aromatic rings include derivatives of biphenyl and naphthalene:

Geminal diamines (1,1-diamines) are an uncommon class of diamines mainly of academic interest. Of the few that exist, most are di-tertiary amines. Bis(dimethylamino)methane ([(CH₃)₂N]₂CH₂) is an isolable example.