Märta Helena Reenstierna (16 September 1753 – 12 January 1841; married name: von Schnell), known as Årstafrun (The Årsta lady), was a Swedish diary writer.

Her diary depicts the details of everyday life of the people of all classes at the estate and its surrounding countryside, the harvests, the tending of animals, household tasks and the weather.

It focuses on the economy and management of the estate but also its everyday life as well as the more festive occasions: its close vicinity to the capital also offers accounts of historical political events as well as news of fashion and pleasure.

She also includes historical political events in nearby Stockholm, which lay near enough to be visited often: for example, she witnessed the punishment of the Armfelt conspirators in 1794, among them Magdalena Rudenschöld.

Märta Helena Reenstierna was portrayed in the novel På Årstafruns tid (English: The times of the Årsta lady) by Lars Widding (1969).