Henry Ustick Onderdonk (March 16, 1789 – December 6, 1858) was the second Episcopal bishop of Pennsylvania.
Onderdonk was born in New York City. 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. 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. 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.
Onderdonk was elected assistant Bishop of Pennsylvania in 1827, serving initially as assistant to Bishop William White. 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.
On Bishop White's death in 1836, Onderdonk succeeded him as bishop. 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.