Othellos Athienou FC won their 1st title.
Fourteen teams participated in the 1990–91 Cypriot Third Division.
The team with the most points at the end of the season crowned champions.
The last three teams were relegated to the 1991–92 Cypriot Fourth Division.
The 3rd-placed team faced the 12th-placed team of the 1991–92 Cypriot Second Division, in a two-legged relegation play-off for one spot in the 1991–92 Cypriot Second Division.