The interview -- the king's first public one in 12 years -- was seen as a bid to repair the monarchy's image, damaged last year by scandals over his elephant-hunting in Africa and a corruption probe implicating his son-in-law.
In the broadcast on national television channel TVE, he reiterated his call for Spaniards to unite in order to pull through the economic crisis that has thrown millions out of work and into poverty.
Millions of families cannot live with dignity and this is making young people have to leave Spain to seek work, to seek what they can. This pains me very much, he said.
I see that Spain has serious problems with the economic crisis, but above all I see a will to move forward despite all that is happening.
He made a tacit swipe at a drive by the Catalonia region to break away from Spain, a by-product of the crisis that has increased tensions, which he branded breakaway politics.
In this time, what Spain needs is unity and for us all to be united.
The king appeared relaxed, seated in his private office in a dark suit and red tie opposite interviewer Jesus Hermida.
Juan Carlos had recently appeared hobbling around on crutches after having both hips replaced, but sympathy for his health was overshadowed by two scandals that raised questions over the monarchy's future.
The first was a corruption case against his son-in-law Inaki Urdangarin, the duke of Palma, who went before a judge in February.
The second was an expensive game-hunting trip the king himself made to Botswana, seen as an unacceptable extravagance while Spain suffered in a recession.
Juan Carlos, who turned 75 yesterday, won wide respect for helping guide Spain to democracy after the death of dictator Francisco Franco in 1975, and helping to quell an attempted military coup in 1981.
A generation after those historic events, a poll published on Thursday showed only half of people expressed a positive judgement of his reign, compared with three-quarters a year ago.
The poll found that nearly 58 percent of people aged between 18 and 29 said they thought a monarchy was not the best form of governance for Spain.
Commentators suggested that people of this generation do not share their parents' reverence for the king since they did not live through the transition and are not very interested in it.
Observers saw the interview as one in a series of efforts to strengthen the monarchy.
The royal palace has launched in recent months a studious marketing operation to improve the king's image, one newspaper wrote in an editorial.
Nearly 45 percent of the poll's respondents said Juan Carlos should abdicate to make way for his son, Felipe, 44. We can have confidence and feel secure and know that in him we have someone who is well prepared, Juan Carlos said of his son.