The Most Rev Bishop Michael Curry, the 27th presiding bishop and primate of the Episcopal Church, will give an address at the wedding in Windsor on May 19.
The preacher, from Chicago, is to attend the service at St George's Chapel, at which the Archbishop of Canterbury will officiate, Kensington Palace said.
The Episcopal Church is an offshoot of the Church of England in the US and forms part of the broader Anglican Communion worldwide.
Ms Markle was baptised by the Most Rev Justin Welby ahead of her wedding to the prince, whose grandmother, the Queen, is head of the Anglican Church.
Bishop Curry said: "The love that has brought and will bind Prince Harry and Ms Meghan Markle together has its source and origin in God, and is the key to life and happiness.
"And so we celebrate and pray for them today."
He is the first African-American bishop to have served as the Episcopal Church's presiding bishop, a role he has held since November 2015.
The Dean of Windsor, David Conner, is to conduct next week's service, before Justin Welby officiates as the couple make their marriage vows.
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