Theodor Undereyck (born 15 June 1635 in Duisburg, died 1 January 1693 in Bremen) was a Protestant pastor, spiritual writer and pioneer of pietism in the German Reformed Church.
After the death of his parents in 1636 by the plague, he grew up as an orphan in the house of his uncle Johann Undereyck in Alstaden.
From 1653 to 1658 he studied Protestant theology in Duisburg, Utrecht, and Leiden, under, among others, Gisbert Voetius and Johannes Cocceius.
In particular Cocceius strongly influenced him in the direction of a federally theological thought, so that he was also considered one of the major catechists of Coccejanism.
Undereyck published 5 books, including two catechisms and a lay dogmatics, in which he conveyed the ideas of the English and Dutch Reformed theology to German-speaking readers in edifying language.