Theodor Christian Petersen Haagaas (15 June 1873, Tistedalen – 25 December 1961, Oslo) was a Norwegian mathematician, mathematics educator and private school owner.

He was described as a distinctive and unconventional teacher with a keen sense of humor, and as the country's best-known mathematics educator.

His mother Nora was born in Norway to Swedish immigrant parents of Forest Finnish heritage, and died early.

[4] In 1908 he married his former pupil Henriette Wegner Paus (1879–1942), a university-educated teacher at Nissen's Girls' School.

[5][6] A blue plaque, placed by Oslo Byes Vel, commemorates his daughter Henriette at Riddervolds gate 9, where the family lived from 1912.

He subsequently studied philosophy, mathematics, natural sciences and law at the Royal Frederick University.

He possessed great authority, was conscientious and meticulous, practical and down-to-earth, patient, clear, and concise.

He was one of the handful of government-appointed examiners in mathematics at the examen artium university entrance exams for several decades.