Whistled speech is a form of speech surrogacy in which whistling is used to mimic speech. Speakers of more than 80 languages have been found to practice various degrees of whistled speech, most of them in rugged topography or dense forests, where movement to carry messages is challenging, and whistling expands the distance of communication. The practice is generally threatened by increased modernization and faster roads, but successful conservation efforts are recorded.

A whistled language is a system of whistled communication which allows fluent whistlers to transmit and comprehend a potentially unlimited number of messages over long distances. Whistled languages are different in this respect from free associative whistling, which may be done to simulate music, to attract attention, or, in the case of herders or animal trainers, to transmit simple messages or instructions to animal companions. Generally, whistled languages emulate the tones or vowel formants of a natural spoken language, as well as aspects of its intonation and prosody, so that trained listeners who speak that language can understand the encoded message.

Whistled language is rare compared to spoken language, but it is found in cultures around the world. It is especially common in tone languages where the whistled tones transmit the tones of the syllables (tone melodies of the words). This might be because in tone languages the tone melody carries more of the functional load of communication while non-tonal phonology carries proportionally less. The genesis of a whistled language has never been recorded in either case and has not yet received much productive study.

Because whistled language is so much rarer than standard vocal language or non-verbal physical language such as sign language, historical research on whistled speech is sparse.

In early China, the technique of transcendental whistling, or xiao, was a kind of nonverbal language with affinities to the spiritual aspects of Daoist meditation. The development of xiao as a practice and art form can be traced through the works of the Western Zhou dynasty, and it was initially used to convey a sense of grief, or to invoke the spirits of dearly departed loved ones. By the time of the Six Dynasties in Han China, xiao had become a widely used complement to spoken language, irrespective of social class. Due to the shrill tones employed while whistling, xiao was often used to punctuate intense feelings or reactions, such as joy, displeasure, and surprise.

In the Melpomene, the fourth book of his Histories, Herodotus makes a passing reference to an Ethiopian tribe who "spoke like bats". While travelling through the territory of an ancient tribe on the southern Black Sea coast in 400 B.C.E, Xenophon wrote in the Anabasis that the Mossynoeci inhabitants could hear one another at great distances across the valleys. The same area encompasses the Turkish village of Kuşköy where whistled speech (kuş dili) is practiced today. Aelian later wrote in De Natura Animalium of the Kinoprosipi people of North Africa, who made use of "acute whistling," who later historians believe were likely a tribe of the Anuak in South Sudan.

In 1982 in the Greek village of Antia on Euboea island, the entire population knew the local whistled speech called sfyria, but only a few whistlers remain now.

Whistled languages have naturally developed in response to the necessity for humans to communicate in conditions of relative isolation, with possible causes being distance, noise levels, and night, as well as specific activities, such as social information, shepherding, hunting, fishing, courtship, or shamanism. Because of this usage, they are mostly related to places with mountains or dense forests. Southern China, Papua New Guinea, the Amazon forest, Subsaharan Africa, Mexico, and Europe encompass most of these locations.