Kasper D(o)enhoff (German: Kaspar von Dönhoff, Polish: Kacper Denhoff, 1587–1645) was a Baltic-German noble (Reichsfürst) of the Holy Roman Empire; a noble (szlachcic) of the Polish-Lithuanian Commonwealth; Governor of Dorpat Province; and a courtier and diplomat of the Polish-Lithuanian Commonwealth.
He became titularly 'Governor of Dorpat' (1627–34) (although already in 1625 the town had capitulated to Sweden and was never reobtained by the Poles); from 1633, Count of the Holy Roman Empire (along with Ernst and Gerhard Dönhoff); Governor of Sieradz (1634–45); court marshal of the Queen (from 1639); and starosta of Wieluń, Lauenburg (Lębork), Radomsko, Bolesławiec, Sokal, Małoszyce, Sobowidze and Klonowo.
A member of the Westphalian, Prussian and Baltic-German von Dönhoff family that also included Ernst Magnus Dönhoff and Marion Dönhoff, Kasper became a military commander (rotmistrz of reiters), a favorite of King Sigismund III Vasa (after his conversion to Catholicism) and one of the most prominent members of the "court faction" that advocated strengthening the Polish monarch's power.
As a courtier of King Władysław IV Vasa, Doenhoff was sent with a diplomatic mission to propose the marriage of Archduchess Cecilia Renata of Austria, daughter of Holy Roman Emperor Ferdinand II, to Władysław IV. The mission was successful, and Kasper also received noble titles from the Holy Roman Emperor.
He was influential and wealthy, enabling him to fund several interesting construction projects, mostly in Sieradz Province. He ordered the rebuilding of a medieval castle at Bolesławiec (of which he was starosta), on the Prosna River, into a new residence, complete with Italian garden. Around 1630 he sponsored a Renaissance-Baroque castle in Kruszyna — the last residential complex in Poland planned around a Renaissance-style internal yard, but already with a Baroque-style grand front yard and garden.
Kruszyna was Doenhoff's main residence; but it was inconveniently located far from the new Commonwealth capital, Warsaw. Hence in 1636 Doenhoff bought and rebuilt another estate, in Ujazd, where he replaced the old castle with a Baroque palace.
In 1620 he married Anna Aleksandra Koniecpolska. They had four children: Aleksander Doenhoff, Stanislaw Doenhoff (?–1653), Zygmunt Doenhoff, and Anna Doenhoff (who married Bogusław Leszczyński).
- Stanisław Ernest Denhoff (1673-1728)
- Kajzer L., "A Sieradz 'Palace Decade'? Kacper Denhoff'S Foundations in the First Half of the 17th Century," Kwartalnik Historii Kultury Materialnej (Quarterly of the History of Material Culture), vol. 52, no. 4, 2004, pp. 403–18.