He assisted a peaceful transition from fascism to democracy in Spain in 1977. But a spate of scandals have tarnished the former king's reputation since.
Now a German aristocrat, Corinna zu Sayn-Wittgenstein – who claims to have had an affair with the former king – says the royal figure used her accounts to launder money and enable illegal financial transactions.
Juan Carlos I abdicated in 2014, two years after he outraged Spaniards by going elephant hunting in Botswana with Sayn-Wittgenstein at the height of the country's recession.
The monarchy's reputation was further tarnished when his youngest daughter Cristina was accused in a corruption probe targeting her husband, who is now in custody.
In audio transcripts published by Spanish daily El Espanol, zu Sayn-Wittgenstein told a former Spanish police commissioner in 2015 that Juan Carlos I had used her status as a Monaco resident to hide undeclared foreign assets and financial transactions.
According to El Espanol's report, the 80-year-old former Spanish king used his cousin, Álvaro Jaime de Orleans-Borbón, to open a series of secret, off-the-radar, accounts in Switzerland.
The Spanish daily's report also accuses a Swiss lawyer, Dante Canonica, of then managing and moving Juan Carlos' reported secret fortune.
Corinna zu Sayn-Wittgenstein alleges that the Swiss accounts were used to launder alleged backhanders the former king received from Spanish companies for helping them obtain lucrative contracts abroad.
One of the key contracts in question is the Medina to Mecca high-speed railway in Saudi Arabia, a €6.3 billion contract awarded to Spanish firms. Zu Sayn-Wittgenstein claims Juan Carlos I used her Monaco-based accounts to deposit his €80 million share from the Saudi Arabian rail deal.
Several of Spain's most senior business figures also feature in Corinna zu Sayn-Wittgenstein's accusations. The Monaco-based financial consultant has also claimed that Spanish secret services and other figures have threatened her for her refusal to hand over key documents – and return the funds.
Spanish political party Podemos has called for the former king to be investigated, although this is unlikely to happen, as he was granted financial amnesty by the former Spanish government of Mariano Rajoy in 2012.
Spain's finance ministry however has ordered the competent authority to open an inquiry into the alleged money laundering affairs of the former king, declaring that nobody is above the law.