Theodore Frelinghuysen Seward (January 25, 1835 – August 30, 1902) was the Founder of the Brotherhood of Christian Unity and the Don't Worry Club.

He first became interested in the tonic sol-fa system during a visit to England in 1869, and on his return worked to introduce the method without adopting the notation.

He then took charge of the performances of the Fisk Jubilee Singers, wrote down more than one hundred of their plantation melodies, and, while making the tour of Europe with them, in 1875–1876.

After a course of study at the Tonic sol-fa college in London, he returned to the United States in 1877, to establish the system.

Besides writing on the subject for many religious and educational journals, and lecturing before gatherings of teachers, he has edited the "Tonic Sol-Fa Advocate" and the "Musical Reform," taught the system in classes and public schools, and prepared a series of textbooks.