Theodoricus Petri Ruutha, also Didrik Persson Ruuta and similar forms of names (* about 1560 in Porvoo; † 1617 in Danzig) was a Finnish administrative lawyer. ''The Piae cantiones'' which he published as a student in 1582 is the first choral book in Scandinavia.

Ruutha attended the cathedral school in Vyborg and studied at the University of Rostock from 1581. On April 7, 1584, he received his graduate degree. He served in the Swedish civil service as bailiff (lappfogd) of Västerbotten from 1595 to 1598, in 1599 of Ångermanland and Ume and in 1600/1601 again of Västerbotten. Around 1600 he explored the borders of Lappland. As a follower of Sigismund III, he was banished from Sweden in 1602 and went to Cracow where he found a job as a secretary at the king's court.

In 1582 he had his collection of songs, Piae cantiones printed in Greifswald which was intended especially for schools. He published this collection together with or on behalf of the rector of the cathedral school in Turku Jacobus Finno. It appeared in a Finnish edition in 1616 and in 1625 at the suggestion of his surviving relatives. It was printed in a second extended edition in Rostock by Daniel Friderici and was reprinted again and again until the end of the 19th century.