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 E₃ 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 that 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. 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 and Russell Oberlin, 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.

In polyphonic compositions of the 14th and early 15th centuries, the contratenor was a voice part added to the basic two-part contrapuntal texture of discant (superius) and tenor (from the Latin tenere, which means to hold, since this part "held" the music's melody, while the superius descanted upon it at a higher pitch). Though having approximately the same range as the tenor, it was generally of a much less melodic nature than either of these other two parts. With the introduction in about 1450 of four-part writing by composers such as Ockeghem and Obrecht, the contratenor split into contratenor altus and contratenor bassus, which were respectively above and below the tenor. Later the term became obsolete: in Italy, contratenor altus became simply altus, in France, haute-contre, and in England, countertenor. Though originally these words were used to designate a vocal part, they are now used to describe singers of that part, whose vocal techniques may differ (see below).

In the Catholic Church during the Renaissance, St. Paul's admonition "mulieres in ecclesiis taceant" ("let the women keep silence in the churches") still prevailed, and women were banned from singing in church services. Countertenors, though rarely described as such, therefore found a prominent part in liturgical music, whether singing a line alone or with boy trebles or altos. (Spain had a long tradition of male falsettists singing soprano lines). Countertenors were hardly ever used for roles in early opera, however, the rise of which coincided with the arrival of a fashion for castrati. For example, the latter took several roles in the first performance of Monteverdi's L'Orfeo (1607). Castrati were already prominent by this date in Italian church choirs, replacing both falsettists and trebles; the last soprano falsettist singing in Rome, Juan [Johannes de] San[c]tos (a Spaniard), died in 1652. In Italian opera, by the late seventeenth century castrati predominated, while in France, the modal high tenor, called the haute-contre, was established as the voice of choice for leading male roles.

In England Purcell wrote significant music for a higher male voice that he called a "counter-tenor", for example, the roles of Secrecy and Summer in The Fairy-Queen (1692). "These lines have often challenged modern singers, who have been unsure whether they are high tenor parts or are meant for falsettists". Contemporary vocal treatises, however, make clear that Purcell's singers would have been trained to blend both methods of vocal production. In Purcell's choral music the situation is further complicated by the occasional appearance of more than one solo part designated "countertenor", but with a considerable difference in range and tessitura. Such is the case in Hail, bright Cecilia (The Ode on St Cecilia's Day 1692) in which the solo, "'Tis Nature's Voice", has the range F₃ to B♭₄ (similar to those stage roles cited previously), whereas, in the duet, "Hark each tree", the countertenor soloist sings from E₄ to D₅ (in the trio "With that sublime celestial lay". Later in the same work, Purcell's own manuscript designates the same singer, Mr Howel, described as "a High Contra tenor" to perform in the range G₃ to C₄; it is very likely that he took some of the lowest notes in a well-blended "chest voice" – see below).

"The Purcell counter-tenor 'tenor' did not flourish in England much beyond the early years of the [eighteenth] century; within twenty years of Purcell's death Handel had settled in London and opera seria, which was underpinned entirely by Italian singing, soon became entrenched in British theatres". In parallel, by Handel's time, castrati had come to dominate the English operatic stage as much as that of Italy (and indeed most of Europe outside France). They also took part in several of Handel's oratorios, though countertenors, too, occasionally featured as soloists in the latter, the parts written for them being closer in compass to the higher ones of Purcell, with a usual range of A₃ to E₅. They also sang the alto parts in Handel's choruses. It was as choral singers within the Anglican church tradition (as well as in the secular genre of the glee) that countertenors survived as performers throughout the 18th and 19th centuries. Otherwise they largely faded from public notice.