Theophil Preiswerk was a Swiss landscape and genre painter.

Theophil Preiswerk was the son of Carl Rudolf Preiswerk, theologian and grammar school teacher, and Sophia nee Fürstenberger.

Preiswerk began his drawing and painting lessons with Ludwig Adam Kelterborn. In 1867 he spent a few months in Charles Gleyre's studio in Paris and studied from November 9, 1867 at the Royal Academy of Arts in Munich and at Karl Otto's private school in Munich. Preiswerk was influenced by the works of Wilhelm Leibl and Hans Thoma exhibited in the Munich Glass Palace. From 1874 to 1876 he stayed with Arnold Böcklin in Florence. From 1877 to 1879 he lived in Rome.

In the 1880s, Preiswerk finally settled in Switzerland, but visited Italy again and again on extensive study trips. From 1883 he occupied a studio in the Kunsthalle Basel for about 15 years. Around 1900, Preiswerk's regular study trips seem to cease. At the same time, he also ended his exhibition activities at the regular and national exhibitions - possibly in response to the repeated accusation that his works were dependent on Arnold Böcklin's art. In 1919, Preiswerk was honored posthumously by his hometown with an exhibition organized by the Basler Kunstverein.

Preiswerk belonged to the Association of Swiss Watercolor Artists.