In poetry, a hendecasyllable (as an adjective, hendecasyllabic) is a line of eleven syllables. The term may refer to several different poetic meters, the older of which are quantitative and used chiefly in classical (Ancient Greek and Latin) poetry, and the newer of which are syllabic or accentual-syllabic and used in medieval and modern poetry.

In classical poetry, "hendecasyllable" or "hendecasyllabic" may refer to any of three distinct 11-syllable Aeolic meters, used first in Ancient Greece and later, with little modification, by Roman poets.

Aeolic meters are characterized by an Aeolic base × × followed by a choriamb – u u –; where – = a long syllable, u = a short syllable, and × = an anceps, that is, a syllable either long or short. The three Aeolic hendecasyllables (with base and choriamb in bold) are:

(Latin: hendecasyllabus phalaecius):

This line is named after Phalaecus, a minor Hellenistic poet who used it in epigrams; though he did not invent it, since it had earlier been used by Sappho and Anacreon.

The Phalaecian hendecasyllable was a favorite of Catullus; it was also very frequently used by Martial. An example from Catullus is the first poem in his collection, given below. The translation attempts to convey some sense of the rhythm, substituting English stress for Latin length.

Cui dōnō lepidum novum libellum
āridā modo pūmic(e) expolītum?
Cornēlī, tibi: namque tū solēbās
meās ess(e) aliquid putāre nūgās

To whom dedicate this, my charming new book,
Freshly burnished with pumice stone to fine sheen?
To Cornelius! you who always used to
Think my gobbledygook was, well, was something.