Decasyllable (Italian: decasillabo, French: décasyllabe, Serbian: десетерац, deseterac) is a type of poetic meter with ten syllables used in traditions that count syllables in verse. In languages with a stress accent (where rhythm depends on stressed syllables), it is similar to pentameter with alternating unstressed and stressed syllables, like iambic pentameter.

Medieval French heroic stories (called chansons de geste) were often written in ten-syllable lines, which became known as "heroic verse." These lines usually had a pause after the fourth syllable. However, medieval French romances (romans) were more commonly written in eight-syllable (octosyllable) lines.

By the 16th century, the use of ten-syllable lines in French poetry was replaced by the 12-syllable alexandrine line. However, Paul Valéry’s famous poem "The Graveyard by the Sea" (Le Cimetière marin) was written using decasyllables.

Similarly, South Slavic and Serbian epic poetry, often sung with a gusle (a string instrument), traditionally uses decasyllabic verse.

In 19th-century Italian opera, decasyllables were frequently used in the libretto (the text of the opera). Musicologist Philip Gossett noted that Giuseppe Verdi asked his librettist, Francesco Maria Piave, to write a chorus for his opera Macbeth with a different rhythm than the famous chorus "Va, pensiero, sull'ali dorate" from Nabucco. Verdi’s request included examples of other operas that used decasyllables, such as I Lombardi and Ernani.

Geoffrey Chaucer, the author of The Canterbury Tales, used decasyllables in his poetry. He adapted this meter into a pattern of alternating stressed and unstressed syllables, known as iambs, which later became famous in the works of Shakespeare. Because Middle English, the language Chaucer used, had many unstressed vowels at the end of words that later became silent, his poetry includes more hendecasyllables (eleven-syllable lines) than modern English poetry.