Ammonihah is a city mentioned in the Book of Mormon. According to the book, the city was founded by an otherwise unknown man named Ammonihah. The inhabitants of Ammonihah were followers of the religion of Nehor.
After Alma the Younger had visited several cities, setting the church in order and preaching, he went to Ammonihah to do the same, but was rejected by the people. As he left the city to preach elsewhere, he saw an angel (the same angel who had confronted him prior to his conversion) and was instructed to return to Ammonihah and preach that the inhabitants of the city would be destroyed unless they repented because the Lord declared: "they do study at this time that they may destroy the liberty of thy people, (for thus saith the Lord) which is contrary to the statutes, and judgments, and commandments which he has given unto his people." On returning, Alma met a resident of Ammonihah named Amulek, who gave Alma food and lodging and joined him in his efforts to preach.
Engaging in a verbal confrontation with a lawyer named Zeezrom, Amulek was able to discern his thoughts by the Power of the Holy Ghost and confound him. Alma then stepped forward and began warning the people that they would be destroyed if they did not repent and believe in the Son of God, even Jesus Christ, and obey his commandments. Many of the inhabitants of Ammonihah were converted through the preaching of Alma and Amulek, but their message was rejected by most of the people. The leaders, lawyers, and judges of the city of Ammonihah then brought Alma and Amulek before their chief judge, accusing them of denouncing their laws and of teaching that God's Son would come among the people but would not save them.
Zeezrom, now convinced of Alma and Amulek's righteousness, started speaking in their defense, but he and the other men who had been converted were driven out of the city, and their wives and children were burnt alive along with their scriptures. Forced to watch, Alma and Amulek were threatened with a similar fate, then imprisoned. After several days of mistreatment in prison, Alma and Amulek were confronted again by the lawyers, teachers, and judges of Ammonihah and challenged to show the power of God. Alma called on God, he and Amulek were freed from their bonds, and the prison tumbled down, killing the city leaders but leaving Alma and Amulek unharmed. When the people rushed to the prison to see what had happened and saw Alma and Amulek standing amidst the ruins, they fled in fear.
Alma and Amulek went to the nearby land of Sidom and found the men who had been expelled from Ammonihah for their belief — including Zeezrom, whom Alma healed of a fever and baptized. A few months later, an invading Lamanite army destroyed the city of Ammonihah and killed all its inhabitants. The dead were piled in a heap and covered with earth; on account of its stench, the site became known as Desolation of Nehors and remained uninhabited for many years.