Truid Aagesen (fl. 1593–1625) was a Danish composer and organist in the service of the Danish king.
Little is known about Aagesen's early life but his musical mentor and spiritual adviser was Jesuit Father Laurentius Nicolai Norvegus with whom he studied music. Aagesen was appointed organist of the Church of Our Lady in Copenhagen on 23 June 1593. He studied in Venice with Giovanni Gabrieli from 1599 to 1600.
In 1600, he went to Prague as a royal commissioner for the Danish king Christian IV. Between 1609 and 1611, he is supposed to have taught at the court and therefore received subsidies from the royal treasury. In 1613, the Danish king published a letter stating that all men of the "popish" religion must leave Denmark. Aagesen, who had been suspected of being on the pope's payroll as early as 1604, was informed of a ruling made in the governing body of Copenhagen University on 15 September 1613 that, since he was popishly inclined, he should not be allowed to continue as organist. In 1615, he was replaced by Johan Meincke. After that he seems to have vanished from history but he is known to have lived in Danzig (Gdańsk) in 1625.
His only known published music is a set of secular Cantiones for three voices which were published in Hamburg in 1608 under his Latinized name, Theodoricus Sistinus. He was also known under the name Trudo Haggaei Malmogiensis.
- Italian Cantiones for three voices (1608)
- Missa Baci amorosi for five voices (unpublished)
- Canon (unpublished)