A shillelagh (/ʃɪˈleɪli, -lə/ shil-AY-lee, -⁠lə; Irish: sail éille or saill éalaigh [ˌsˠal̠ʲ ˈeːlʲə], 'thonged willow') is a wooden walking stick and club or cudgel, typically made from a stout knotty blackthorn stick with a large knob at the top. It is associated with Ireland and Irish folklore.

Other spelling variants include shillelah, shillalah, and shillaly.

The name shillelagh is the Hiberno-English corruption of the Irish (Gaelic) form sail éille, where sail means 'willow' or 'cudgel' and éille is genitive for iall meaning 'thong', 'strap', 'leash', and 'string', among others.

As an alternate etymology, Anna Maria Hall and Patrick Weston Joyce have written that the name may have been derived from the wood being sourced from forest land in the village or barony of Shillelagh, County Wicklow. The geographic name Shillelagh derives from Síol Éalaigh, or 'descendants of Éalach' in English.

Shillelaghs are traditionally made from blackthorn (sloe) wood (Prunus spinosa) or oak. With the scarcity of oak in Ireland the term came increasingly to denote a blackthorn stick, and therefore blackthorn stick is sometimes glossed as an equivalent to shillelagh.

Wood from the root was prized since this would be used for the knob, and was less prone to crack or break during use.

Most commonly, the chosen wood would be placed up a chimney to cure for a duration of several months to several years; the accumulated layer of soot gave the shillelagh its typical black shiny appearance.

The less frequent methods were to bury the shank in a dung pile, or in slaked lime. The stick may require protection from its dung bath by being wrapped in well-greased oiled brown paper (steeped in hog's lard or oil).