In enzymology, a guanidinoacetate kinase (EC 2.7.3.1) is an enzyme that catalyzes the chemical reaction

ATP + guanidinoacetate ${\displaystyle \rightleftharpoons }$ ADP + phosphoguanidinoacetate

Thus, the two substrates of this enzyme are ATP and guanidinoacetate, whereas its two products are ADP and phosphoguanidinoacetate.

Guanidinoacetate kinase belongs to the family of transferases, specifically those that transfer phosphorus-containing groups (phosphotransferases) with a nitrogenous group as acceptor. The systematic name of this enzyme class is ATP:guanidinoacetate N-phosphotransferase. This enzyme is also called glycocyamine kinase. This enzyme participates in arginine and proline metabolism.