In enzymology, a homospermidine synthase (spermidine-specific) () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are spermidine and putrescine, whereas its two products are sym-homospermidine and propane-1,3-diamine.
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 spermidine:putrescine 4-aminobutyltransferase (propane-1,3-diamine-forming).