The junk rig, also known as the Chinese lugsail, Chinese balanced lug sail, or sampan rig, is a type of sail rig in which rigid members, called battens, span the full width of the sail and extend the sail forward of the mast. While relatively uncommon in use among modern production sailboats, the rig's advantages of easier use and lower maintenance for blue-water cruisers have been explored by individuals such as trans-Atlantic racer Herbert "Blondie" Hasler and author Annie Hill.

The English word "junk" comes from Portuguese junco from Malay jong. The word originally referred to (with or without junk rigs) the Javanese djong, very large trading ships that the Portuguese first encountered in Southeast Asia. It later also included the smaller flat-bottomed Chinese chuán, even though the two were markedly different vessels. After the disappearance of the jong in the 17th century, the meaning of "junk" (and other similar words in European languages) came to refer exclusively to the Chinese ship.

The origin of the junk sailing rig is not directly recorded. The Chinese adopted the sail design from other cultures, although the Chinese made their own improvements over time. Paul Johnstone attribute the invention of this type of sail to Austronesian peoples from Indonesia. They were originally made from woven mats reinforced with bamboo, dating back to at least several hundred years BCE. They may have been adopted by the Chinese after contact with Southeast Asian traders (K'un-lun po) by the time of the Han dynasty (206 BCE to 220 CE). However, Chinese vessels during this era were mostly fluvial (riverine) while others were made to cross shorter distances over the seas (littoral zones); China did not build true ocean-going fleets until the 10th century Song dynasty. The Chinese were using square sails during the Han dynasty; only in the 12th century did the Chinese adopt the Austronesian junk sail.

Sinologist Joseph Needham also argues that Chinese balanced lug sails developed from Indonesian tilted sails. Iconographic remains show that Chinese ships before the 12th century used square sails. A ship carving from a stone Buddhist stele shows a ship with square sail from the Liu Sung dynasty or the Liang dynasty (ca. 5th or 6th century). Dunhuang cave temple no. 45 (from the 8th or 9th century) features large sailboats and sampans with inflated square sails. A wide ship with a single sail is depicted in the Xi'an mirror (after the 9th or 12th century).

Needham thinks a ship in the Borobudur relief may have been the first to depict a junk sail. The ship is distinct from the reliefs of other ships and could be the oldest depiction of a Chinese seafaring ship. This is disputed by D.A. Inglis, who concluded that the ship described was more like Indian Ocean ships operating from Arabia and South Asia after conducting an on-the-spot investigation of the relief. The ship has protruding deck beams, a single mast (not a bipod or tripod), and a square sail that has a yard and boom.

The oldest depiction of a battened junk sail comes from the Bayon temple at Angkor Thom, Cambodia. This depiction may feature a Chinese ship, but this is disputed by Nick Burningham, who points out that the ship had a keel and that Chinese ships generally did not have a sternpost (extension of the keel at the rear of the ship). The rudder has a thin blade, different from the Chinese rudder which usually has a long blade. These characteristics may suggest a quarter rudder mounted on the beams of the rear gallery. From its characteristics and location, it is likely that this ship is a Southeast Asian ship.

As the origins of the junk sail remain unknown, no particular rig can claim to be the original or correct way to rig a junk sail. Some sailors have demonstrated the junk sails ability to work even in the presence of some standing rigging, such as the Colvin rig, although more care must be taken to prevent damage while sailing.

Some ships that have been known to use junk sails include: