In biochemistry, a ligase is an enzyme that can catalyse the joining of two molecules ("ligation" or "glue together") by forming a new chemical bond, with concomitant hydrolysis of ATP or other similar molecules. They are classified as EC 6 in the EC number classification. Many ligases are also known as "synthases" or "synthetases" because they synthesize new molecules.

In a molecular biology laboratory, one of the most frequently used ligase is DNA ligase. It is used to connect DNA fragments together.