A G-string or thong is a narrow piece of cloth or leather, or plastic that passes between the buttocks and is attached to a band around the hips, worn as a swimwear or underwear by both men and women. The two terms, g-string and thong are often used interchangeably however, the primary difference between the two garments is that a g-string has less material around the waist band and between the buttocks hence, a string-like appearance.

The G-string or thong is probably the earliest form of clothing known to mankind; having originated in the warmer climates of sub-Saharan Africa where clothing was first worn nearly 75,000 years ago. Many tribal peoples, such as some of the Khoisan people of southern Africa, wore thongs for many centuries. Much like the 2000-plus-year-old Japanese fundoshi, these early garments were made with the male genitalia in mind.

Although developed for the male anatomy by primitive peoples, in the modern West thongs are more often worn by females. They first gained mainstream popularity as swimwear in South America, particularly in Brazil in the 1970s. In Brazil, where the buttocks ("bunda" in Portuguese) are especially admired and emphasized; it was originally a style of swimsuit whose rear area became so narrow that it would disappear between the wearer's buttocks.

The origin of the term "G-string" is obscure. The term is first attested in writings by Americans in the late 1800's regarding the loincloth of Philippine natives. Others attest it to being the g-string on an instrument. The origin of the word "thong" is from the Old English thwong, a flexible leather cord.