The Sphinx-class sailing sixth rates were a series of ten post ships built to a 1773 design by John Williams. Although smaller than true frigates, post ships were often referred to incorrectly as frigates by sea officers, but not by the Admiralty or Navy Board.

Class overview
Name	Sphinx-class post ships
Operators	Royal Navy
Completed	10
General characteristics
Type	Sixth-rate post ship
Tons burthen	431 37/94 (as designed)
Length	108 ft (33 m) (gundeck) 89 ft 7.375 in (27.31453 m) (keel)
Beam	30 ft 1 in (9.17 m)
Depth of hold	9 ft 8 in (2.95 m)
Propulsion	Sail
Sail plan	Full-rigged ship
Complement	140 (reduced to 134 in 1794).
Armament	UD: 20 × 9-pounder guns QD (added 1794): 4 × 12-pounder carronades FC (added 1794): 2 × 12-pounder carronades

The first vessel in the class was launched in 1775, six more in 1776, two in 1777 and the last in 1781. The vessels of the class served in the Royal Navy during the American Revolutionary War. Three of them - Sphinx and Ariel in September 1779, and Unicorn in September 1780 - were captured by the French Navy, but Sphinx was recovered in December 1779 and Unicorn in April 1781. Some survived to see service in the French Revolutionary and Napoleonic Wars.