In enzymology, an RNA ligase (ATP) (EC 6.5.1.3) is an enzyme that catalyzes the chemical reaction

ATP + (ribonucleotide)n + (ribonucleotide)m ${\displaystyle \rightleftharpoons }$ AMP + diphosphate + (ribonucleotide)n⁺m

The 3 substrates of this enzyme are ATP, (ribonucleotide)n, and (ribonucleotide)m, whereas its 3 products are AMP, diphosphate, and (ribonucleotide)n+m.

This enzyme belongs to the family of ligases, specifically those forming phosphoric-ester bonds. The systematic name of this enzyme class is poly(ribonucleotide):poly(ribonucleotide) ligase (AMP-forming). Other names in common use include polyribonucleotide synthase (ATP), RNA ligase, polyribonucleotide ligase, and ribonucleic ligase.

As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes 1VDX and 2C5U.