Sector 5 was a region of space. A portion of the Neutral Zone was located in this sector.

In 2286, the USS Saratoga was patrolling sector 5 when it began tracking a large unknown alien probe on a trajectory to the Terran solar system. (Star Trek IV: The Voyage Home)

According to Star Trek: Star Charts (p. 19), Sector 5 was also known as the Vulcan Sector. This sector contained the Vulcan system.