A countertenor (also contra tenor) is a type of classical male singing voice whose vocal range is equivalent to that of the female contralto or mezzo-soprano voice types, generally extending from around G₃ to D₅ or E₅, although a sopranist (a specific kind of countertenor) may match the soprano's range of around C₄ to C₆. Countertenors often have tenor or baritone chest voices, but sing in falsetto or head voice much more often than they do in their chest voice.

The nature of the countertenor voice has radically changed throughout musical history, from a modal voice, to a modal and falsetto voice, to the primarily falsetto voice which is denoted by the term today. This is partly because of changes in human physiology (increase in body height) and partly because of fluctuations in pitch.

The term first came into use in England during the mid-17th century and was in wide use by the late 17th century. However, the use of adult male falsettos in polyphony, commonly in the soprano range, was known in European all-male sacred choirs for some decades previous, as early as the mid-16th century. Modern-day ensembles such as the Tallis Scholars and the Sixteen have countertenors on alto parts in works of this period. There is no evidence that falsetto singing was known in Britain before the early 17th century, when it was occasionally heard on soprano parts.

In the second half of the 20th century, there was great interest in and renewed popularity of the countertenor voice, partly due to pioneers such as Alfred Deller, as well as the increased popularity of Baroque opera and the need of male singers to replace the castrati roles in such works. Although the voice has been considered largely an early music phenomenon, there is a growing modern repertoire collection for countertenors, especially in contemporary music.