The 85-year-old king emeritus, who abdicated in favor of his son, Felipe VI, in June 2014, is attending a sailing regatta in the resort of Sanxenxo in the north-western region of Galicia.
Juan Carlos flew to Spain from London in a private jet. On Tuesday, he watched Real Madrid's 2-0 win against Chelsea in the Champions League in London, and he was also pictured leaving an exclusive club there on Monday night.
The former King is not expected to travel to Madrid or meet his son during what was described as a "private visit" by Spanish government spokesperson Isabel Rodriguez.
Juan Carlos has been credited by some with helping Spain's "transition" after the death of Gen. Francisco Franco in 1975, but his image has been tainted in recent years by financial scandals and controversies in his personal life.
In 2020, an investigation was launched into suspicions that he was paid kickback by Spanish firms to secure a contract to build a high-speed rail link between Medina and Mecca in Saudi Arabia.
That investigation was closed in 2022, due partly to statute limitations and to the fact that Juan Carlos is immune from prosecution for actions pertaining to the period before his abdication.