Appearing in full military garb alongside his son, Queen Letizia and his wife Sofia at the royal palace in Madrid, Juan Carlos met Spain's army, navy and airforce chiefs at a traditional military ceremony held at the start of every year.
The show of unity comes at a trying time for Spain after the wealthy Catalan region attempted to break away, and where 47 percent of voters want to live in an independent, monarchy-free republic.
It also puts Juan Carlos, credited for leading Spain to democracy after decades of dictatorship but who abdicated in 2014 after a series of scandals, back on the public scene.
Felipe congratulated his father on turning 80 on Friday, drawing a smile from the elderly former monarch, who now walks with a stick.
"Congratulations your majesty and thank you also for so many years of loyal service to Spain," he said, as Prime Minister Mariano Rajoy and his defence and interior ministers looked on.
Role in 'democratic' Spain
Widely lauded for his negotiating skills, political acumen and his role in defusing an attempted coup in 1981, Juan Carlos fell from grace during Spain's economic crisis as his lavish lifestyle drew anger.
Adding fuel to fire, his daughter Princess Cristina and her husband Inaki Urdangarin were embroiled in a corruption and tax fraud investigation, with the latter since found guilty of siphoning off millions to fund a lavish lifestyle. Cristina was acquitted.
When he took the throne, Felipe VI, who turns 50 at the end of the month, kept his distance in a bid to restore the image of the monarchy.
In June, Juan Carlos was notably absent from an event marking 40 years since the first democratic elections after Francisco Franco's dictatorship, despite having been widely credited for enabling the vote to take place.
That, according to Felipe VI's biographer and other royal affairs experts, angered and hurt him.
"It was thanks to him that there was an exemplary transition from a dictatorship to a full democracy," said Jaime Penafiel, a royal affairs journalist.
"He made it known that if they humiliated him again, he would go and celebrate his 80th abroad."
So it was that on Saturday, Juan Carlos and Sofia accompanied Felipe VI and Queen Letizia to the ceremony for the first time.
Defence Minister Maria Dolores de Cospedal credited Juan Carlos for being instrumental in building a "democratic and modern Spain... from the basis of reconciliation, harmony and peaceful cohabitation".
Four years ago, the same ceremony had been torture for Juan Carlos, physically weakened after health issues and tainted by scandals.
"He made mistakes in his speech, lost track of what he was saying, offering a pathetic spectacle," said Jose Apezarena, Felipe VI's biographer.
That was when he asked for preparations to be made for his abdication, he said.
Later that year in June, he stepped down in favour of his son and became "king emeritus," a title he hates according to the press.
"It wasn't what he wanted as he had repeatedly said he would be king until he died," said Penafiel.
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