Sietske (variations include Sytske) is a Frisian girls' name, which is mostly found in the northern regions of the Netherlands.
Sietske is based on the stemm "Siet". It is possible that it derives from Old Frisian "side", meaning "morale" or "sîth" meaning "companion". It is also possible it is a shortened form, or nickname for older Germanic starting with "sigi-" or "sî-" meaning "victory"
The name Sietske is a girl's name, while Sietse is a boy's name. Outside the Dutch province of Friesland this convention is not well known. Therefore the greater part of the Sietskes from provinces outside Friesland are wrongfully addressed as "Sir" in written correspondence.