NGC 6558 is a globular cluster, located about 24,000 light years away in the constellation Sagittarius. Its apparent magnitude is about 11 and its apparent diameter is about 10 arcminutes.^[2] The globular cluster was discovered in 1784 by the astronomer William Herschel with his 18.7-inch telescope and the discovery was later catalogued in the New General Catalogue.

NGC 6558
The globular cluster NGC 6558 imaged by the Hubble Space Telescope
Observation data (J2000 epoch)
Class	V
Constellation	Sagittarius
Right ascension	18h 10m 18.38s[1]
Declination	−31° 45′ 48.6″[1]
Distance	24.1 kly (7.4 kpc)[1]
Apparent magnitude (V)	11.29[1]
Physical characteristics
Radius	5.2' x 5.2'[2]
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = -1.32[3] dex
Other designations	Cr 368, GCl 89, ESO 456-62, VDBH 259[1]
See also: Globular cluster, List of globular clusters

It is located 1.5 degrees south-southeast of Gamma² Sagittarii.