Juan de Cartagena (died c. 1520) was a Spanish aristocrat who served on the Magellan expedition as the inspector general of the fleet and captain of one of the five ships sent by Spain to find a western route to Asia.

In councils between captains, Cartagena routinely opposed Magellan's navigation decisions and refused to salute his superior when required by custom to do so.

A storm delayed the fleet south of Tenerife, and food had to be rationed; Cartagena took this opportunity to publicly criticise Magellan and suggest he was not competent to command.

On 1 April 1520 he secretly left the Victoria and reboarded the San Antonio, where he rallied supporters among the Spanish crew and officers in opposition to the Portuguese Magellan.

In company with Concepción's captain Gaspar de Quesada, pilot Juan Sebastián Elcano and thirty Spanish crew members, Cartagena seized control of San Antonio and declared the vessel independent of Magellan's command.

The officers of both Concepción and Victoria joined in the mutiny, and on 2 April 1520 a letter was sent to Magellan's flagship, the Trinidad, demanding that the captain-general acknowledge that the fleet was no longer under his command.

However, Magellan was reluctant to execute a close relative of Fonseca,[11] so Cartagena was sentenced to be marooned along with another conspirator, the priest Pedro Sánchez de la Reina.