In enzymology, a sym-norspermidine synthase (EC 2.5.1.23) is an enzyme that catalyzes the chemical reaction

S-adenosylmethioninamine + propane-1,3-diamine ${\displaystyle \rightleftharpoons }$ 5'-methylthioadenosine + bis(3-aminopropyl)amine

Thus, the two substrates of this enzyme are S-adenosylmethioninamine and propane-1,3-diamine, whereas its two products are 5'-methylthioadenosine and bis(3-aminopropyl)amine.

This enzyme belongs to the family of transferases, specifically those transferring aryl or alkyl groups other than methyl groups. The systematic name of this enzyme class is S-adenosylmethioninamine:propane-1,3-diamine 3-aminopropyltransferase. This enzyme participates in urea cycle and metabolism of amino groups.