Boyd Packer, head of the Mormon Church's highest governing body, died at his home in Salt Lake City on Friday, the church said on its website.
He was 90 and died of age-related causes, Elder Russell Ballard of the Church of Jesus Christ of Latter-day Saints said in a statement.
Packer, who served as president of the church's Quorum of the Twelve Apostles, had been next in line to be church president, according to his obituary in the Salt Lake Tribune.
The Salt Lake City-based Mormon Church says its members number more than 15 million people around the world.
Packer was a frequent spokesman for his faith's conservative values.
In April, after Utah enacted its first nondiscrimination protections for gay and transgender residents, Packer spoke out to extol the eternal value of marriage between a man and a woman.
Faithful members of the LDS Church believe that “the only legitimate, authorized expression of the powers of procreation is between a husband and wife who have been legally and lawfully married," Packer said.
The Salt Lake Tribune said that Packer preached in 2010 that gays could resist the urge to act on their attractions and suggested they could change their sexual orientation.
For the church, Packer worked on new editions of its history and scriptures, the LDS statement said.
“President Packer always felt that if we could read the words of the Lord we would be far better off and much safer than speculating with our own ideas," Ballard said. "It was not unusual for him to say, ‘Brethren, let me read to you.’"
Ballard called Packer a dedicated "apostle ... from the crown of his head to the soles of his feet."
Born in Brigham City, Utah, Packer served as a U.S. bomber pilot during World War II.
He is survived by his wife, Donna, and their 10 children.