A sejmik (diminutive of the Polish sejm, or parliament) was a regional sejm in the pre-partition Polish-Lithuanian Commonwealth, and earlier in the Kingdom of Poland.

There were three kinds of sejmik:

• general, held in western Poland (Greater Poland) at Kolo, in southern Poland (Little Poland) at Nowe Miasto Korczyn, in Mazovia (Polish: Mazowswze) at Warsaw, in Red Ruthenia at Sadowa Wisznia , and in Lithuania at Volkovysk ;
• provincial (in every voivodship); and
• county (powiat).

They could meet for special purposes:

• Pre-sejm (Polish: przedsejmowe) sejmiks elected deputies (poslowie) to the ordinary General Sejm (Polish: Sejm Walny) that was held every two years for six weeks, and to any extraordinary General Sejm that might be called at any time in an emergency, to sit for two weeks.
• Relational (Polish: relacyjne) sejmiks heard the relations of deputies returned from the General Sejm.
• Deputational (Polish: deputackie) sejmiks elected deputies (deputaci) to general tribunals.
• Administrative (or economic, Polish: gospodarcze) sejmiks oversaw voivodship self-government.
• Electoral (Polish: elekcyjne) sejmiks elected higher voivodship officials.

Sejmiks were called by the king or the provincial governor (voivod). All the nobility (szlachta) residing in a voivodship were eligible to participate in sejmiks; other estates sent only delegations with an advisory voice. Each sejmik elected a sejmik marshal ( marszalek sejmiku: presiding officer of the sejmik). A sejmik could be disrupted, which eventuality was guarded against by the formation (as at sejms) of a confederation, which obliged majority voting.

In the 16th century the leading force at sejmiks was the middle nobility; later this passed to the magnates (magnaci). Sejmiks attained a peak of importance at the turn of the 17th and 18th centuries, when they often set their own time-limits — that is, extended their authorized periods of operation. Such abuses were suppressed by acts of the one-day Silent Sejm (Polish: sejm niemy) of 1717.

See also:

• Confederated sejm
• Liberum veto
• May Constitution of Poland (adopted 3 May 1791)
• Voivodships of Poland