Henry Ustick Onderdonk (March 16, 1789 – December 6, 1858) was the second Episcopal bishop of Pennsylvania.

Onderdonk was born in New York City.^[1] He studied at Columbia University, receiving his degree in 1805, and then traveled to Britain for further education, receiving his medical degree from the University of Edinburgh.^[1] On returning to the United States, Onderdonk practiced medicine in New York before being ordained to the deaconate and priesthood by Bishop John Henry Hobart.^[1] In 1816, he went to western New York as a missionary and then returned east to become rector of St. Ann's Church in Brooklyn, remaining there for seven years.^[1]

Onderdonk was elected assistant bishop of Pennsylvania in 1827, serving initially as assistant to Bishop William White.^[2] He was the 21st bishop of the ECUSA, and was consecrated by bishops William White, Alexander Viets Griswold, and James Kemp. However, bishop Kemp died of injuries received in a stage coach accident while returning from the consecration, so Onderdonk substituted in the Episcopal Diocese of Maryland until a successor was elected.^[3] In 1830, Onderdonk published Episcopacy Tested In Scripture, first published in the Protestant Episcopalian and then as tract by the Protestant Episcopal Tract Society, a defense of episcopacy based "on an appeal to the bible alone."^[4]

On Bishop White's death in 1836, Onderdonk succeeded him as bishop.^[2] Onderdonk was a strong advocate of the pre-Tractarian High Church position, in company with his brother Benjamin Treadwell Onderdonk, who was also a bishop. When Rev. Alexander Crummell petitioned to be allowed to move to Pennsylvania to establish another church (besides the peripatetic St. Thomas congregation) to serve Philadelphia's African-American community, Bishop Onderdonk reportedly replied, "I will receive you into this diocese on one condition: No negro priest can sit in my church convention and no negro church must ask for representation there." Crummell reportedly paused for a moment before declining.^[5]

In 1844, Onderdonk was suspended from the exercise of his Episcopal office after rumors of alcoholism.^[2] The suspension was lifted in 1856, two years before his death.^[2]

He is buried in the churchyard of Church of St. James the Less in Philadelphia.