In enzymology, a pyrimidine-nucleoside phosphorylase (EC 2.4.2.2) is an enzyme that catalyzes the chemical reaction

a pyrimidine nucleoside + phosphate ${\displaystyle \rightleftharpoons }$ a pyrimidine base + alpha-D-ribose 1-phosphate

Thus, the two substrates of this enzyme are pyrimidine nucleoside and phosphate, whereas its two products are pyrimidine base and alpha-D-ribose 1-phosphate.

This enzyme belongs to the family of glycosyltransferases, specifically the pentosyltransferases. The systematic name of this enzyme class is pyrimidine-nucleoside:phosphate alpha-D-ribosyltransferase. This enzyme is also called Py-NPase. This enzyme participates in pyrimidine metabolism.

As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes 1BRW and 2DSJ.