Otto Dirk Duintjer (Amsterdam, 30 April 1932-Heemstede, 23 November 2020) was a Dutch philosopher. He studied Theology at the Free University in Amsterdam (VU) and the University of Amsterdam (UvA), as well as Philosophy at Groningen University. He was an Assistant Professor at Leiden University from 1960 to 1970, where he obtained his doctorate in Philosophy in 1966 (cum laude) with a dissertation entitled The Issue of the Transcendental, Especially in Relation to Heidegger and Kant (in Dutch).
From 1970 to 1987, he was Full Professor of Epistemology and Metaphysics at the University of Amsterdam. From 1987 until his retirement in 1997, he held the chair of “Philosophy and Spirituality” at the same university.
In 2004, he received an honorary doctorate from the Universiteit voor Humanistiek in Utrecht. He was one of the founding members of the Foundation for Philosophy East and West (Stichting Filosofie Oost-West), which organizes philosophy courses for the general audience, and a long-time member of its Program Board.