Brantz Mayer (September 27, 1809 - February 23, 1879) was an American author.
He was born in Baltimore to Christian Mayer, a German who emigrated in 1784 along with a friend, Lewis Brantz. Forming a business partnership that would last for decades, Lewis Brantz and the elder Mayer engaged in trade with the East Indies and Mexico. As well, Mayer for many years served as consul general of Würtemberg in the United States.
In 1809, Christian Mayer had a second son (the first was lawyer Charles F. Mayer), whom he named after his friend and business partner. Childless himself, Lewis Brantz would eventually name the younger Mayer as his heir.
After graduating at St. Mary's College, Baltimore, Brantz Mayer sailed for the East, visiting Java, Sumatra, and China, and returned in 1828. He studied law during this long voyage, and on his return home he entered the law school of the University of Maryland and was admitted to the bar in 1829. He practiced law from 1832 until 1841, when he was appointed secretary of legation to Mexico, where he remained a year, and on his return edited for a short time the Baltimore American newspaper. Brantz Mayer was again secretary of the United States legation to Mexico in 1842 and 1843.