Johann Theodor Gsell Fels was a Swiss non-fiction author, art historian, and physician.

Gsell, son of merchant and lithographer Jakob Laurenz Gsell, originally wanted to become a pastor. He therefore first studied Protestant Reformed theology and philology at the University of Basel for two and a half years. He won first prize in a philosophy competition there. He then continued his studies at the University of Berlin under Friedrich Strauss and Franz Theremin, where he also attended the philosophy lectures of Friedrich Wilhelm Joseph Schelling with particular interest. He then gave up his plan to become a pastor and studied art history in Berlin under Franz Kugler and Heinrich Gustav Hotho. To supplement his studies, he then undertook extensive educational journeys on foot throughout Italy.

From 1845 to 1848, he studied natural sciences at the University of Paris. After returning to St. Gallen in 1848, he took up a position in the state archives there. He then studied medicine in Würzburg, Vienna, and Berlin. After receiving his doctorate in medicine in 1856, he practiced as a physician in Nice, Rome, Paris, and Zurich. In 1870, Gsell settled in Basel. He worked at the university there as a lecturer in Italian art history and was a member of the Grand Council. In 1880, he moved to Munich. He wrote numerous travel and art guides, including ones on Italy and Switzerland, most of which were reprinted several times, in revised form even a quarter of a century after his death. Some of his travel guides were translated into English.

In 1850, Gsell married Louise Carolina Fels, the daughter of St. Gallen politician Christian Friedrich Fels. In 1883, he officially changed his family name to Gsell Fels by a decision of the St. Gallen cantonal government.

Theodor Gsell Fels' brother, Gaspard Gsell, made a name for himself in France as a glass painter.

Theodor Gsell-Fels died in Munich in 1898 at the age of 80.