In enzymology, a homoserine kinase (EC 2.7.1.39) is an enzyme that catalyzes the chemical reaction

ATP + L-homoserine ${\displaystyle \rightleftharpoons }$ ADP + O-phospho-L-homoserine

Thus, the two substrates of this enzyme are ATP and L-homoserine, whereas its two products are ADP and O-phospho-L-homoserine.

This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing groups (phosphotransferases) with an alcohol group as acceptor. The systematic name of this enzyme class is ATP:L-homoserine O-phosphotransferase. Other names in common use include homoserine kinase (phosphorylating), and HSK. This enzyme participates in glycine, serine and threonine metabolism.

As of late 2007, 6 structures have been solved for this class of enzymes, with PDB accession codes 1FWK, 1FWL, 1H72, 1H73, 1H74, and 2PPQ.