| NGC 6539 | |
| Epoch: | J2000 |
| Class: | X: |
| Constellation: | Serpens |
| Dist Ly: | 8.165kpc |
| Appmag V: | 9.6 |
| Size V: | 6.9 |
| Absmag V: | −8.21 |
| Metal Fe: | −0.66 |
| Names: | GCL 85 |
NGC 6539, or GCL 85, is a globular cluster of stars in the constellation Serpens. It was discovered by Danish astronomer Theodor Brorsen in 1856. This cluster is visible with a small amateur telescope, having an apparent visual magnitude of 9.6 and an angular size of . It is located at a distance of 8.165kpc from the Sun, and 3.1kpc from the Galactic Center.
This cluster is located in the outer parts of the Galactic bulge. There is a large dark cloud complex located in the fore of this cluster, causing reddening from extinction. NGC 6539 has a core radius of, a half-mass radius of, and a tidal radius of . Compared to other globular clusters, NGC 6539 is considered metal rich, indicating that it has a relatively higher abundance of elements more massive than helium. Thirteen variable stars have been detected within of the core, of which a dozen are long period variables. A millisecond pulsar was discovered in 1990.