In enzymology, a cysteine—tRNA ligase (EC 6.1.1.16) is an enzyme that catalyzes the chemical reaction

ATP + L-cysteine + tRNACys ${\displaystyle \rightleftharpoons }$ AMP + diphosphate + L-cysteinyl-tRNACys

The 3 substrates of this enzyme are ATP, L-cysteine, and tRNA(Cys), whereas its 3 products are AMP, diphosphate, and L-cysteinyl-tRNA(Cys).

This enzyme belongs to the family of ligases, to be specific those forming carbon-oxygen bonds in aminoacyl-tRNA and related compounds. The systematic name of this enzyme class is L-cysteine:tRNACys ligase (AMP-forming). Other names in common use include cysteinyl-tRNA synthetase, cysteinyl-transferRNA synthetase, cysteinyl-transfer ribonucleate synthetase, and cysteine translase. This enzyme participates in cysteine metabolism and aminoacyl-trna biosynthesis.

As of late 2007, 3 structures have been solved for this class of enzymes, with PDB accession codes 1LI5, 1LI7, and 1U0B.