Peter Debye was described as a martinet when it came to scientific principles, yet he was always approachable and made time for his students. His personal philosophy emphasized a fulfillment of purpose and enjoyment in one's work. Debye was an avid trout fisherman and gardener, collector of cacti, and was "always known to enjoy a nice cigar".
While in Berlin as an assistant to Arnold Sommerfeld, Debye became acquainted with Mathilde Alberer. Mathilde was the daughter of the proprietor of the boarding house in which Debye was staying. Mathilde would soon change her citizenship and in 1913, Debye married Mathilde Alberer. Debye would enjoy working in his rose garden with Mathilde Alberer late into his years. They had a son, Peter P. Debye (1916–2012), and a daughter, Mathilde Maria (1921–1991). Peter became a physicist and collaborated with Debye in some of his research, and had a son who was also a chemist.
Debye was a faithful Catholic who insisted his family go to church.