After heartbreak in Beijing, London, Rio de Janeiro and Tokyo, Djokovic simply would not be denied the one title that had eluded him so long, winning 7-6(3) 7-6(2) in front of an enthralled crowd on Court Philippe Chatrier.
Top seed Djokovic had to deliver one of the finest performances of his career to deny a man 16 years his junior and become only the fifth player to win all four singles Grand Slams and the Olympic title during their careers.
"It was an incredible fight and I had to play my best tennis," an emotional Djokovic said on court as flag-waving Serbian fans sang his name. "I put my heart, my soul, everything to win gold. I did it for my country first, for Serbia."
In a raucous atmosphere, with soccer-style chants shattering the calm of the adjacent Bois de Boulogne, neither player took a backward step in a ferocious contest.
The first set alone lasted one hour 33 minutes as the two wrestled for control in a series of spellbinding games, one at 4-4 in which Djokovic repelled five break points.
Alcaraz blinked first in the tiebreak to allow Djokovic to move ahead and when another tiebreak was required to decide the second set, again Djokovic found inspiration, moving 6-2 in front and sealing victory with a stunning forehand winner.
Djokovic roared to the sky and after consoling a crestfallen Alcaraz at the net, then fell to his knees and sobbed into the Parisian clay in the centre of the court before climbing into the crowd where he was swamped by his family, friends and team.
While Djokovic became the oldest Olympic singles champion since tennis returned to the Games in 1988, the 21-year-old Alcaraz looked inconsolable after failing to add the gold medal to this year's French Open and Wimbledon crowns.
"I think I did a really great tournament, I think I did a really great match," the four-time Grand Slam champion said. "Obviously in front of me, I had a really hungry Novak."
Djokovic had lost three times in Olympic singles semi-finals with a solitary bronze medal to show from four previous Games.
He knew Paris was his last realistic chance to fill the remaining space in a bulging trophy cabinet containing 24 Grand Slam titles. And how he earned it -- overturning a crushing loss to Alcaraz three weeks ago in the Wimbledon final to become the first man to win the Olympic singles title without dropping a set.
INCREDIBLE FIGHT
Djokovic joins an elite group of Steffi Graf, Andre Agassi, Rafa Nadal and Serena Williams, the latter watching from the front row, in the Golden Slam club, and many will say this settled the G.O.A.T argument once and for all.
It takes something pretty special to upstage the men's 100 metres final taking place later at the Stade de France, but this near three-hour red-dirt duel arguably came close.
The opening four games spanned 30 mesmerizing minutes, every rally a battle of skill and wills. Alcaraz saved three break points in the fourth game and looked menacing as he threw everything in his arsenal at Djokovic as the Serb served at 4-4.
Several times Djokovic could only smile at the athleticism of his opponent who piled on the pressure to earn five break points, and somehow he survived.
Alcaraz saved a set point at 5-6 but the Spaniard's level dipped in the tiebreak and Djokovic pounced to grab the lead. Sensing his moment, Djokovic's intensity went even higher deep in the second set but Alcaraz hung on.
Djokovic was relentless though and an outrageous forehand winner at 2-2 in the breaker snapped Alcaraz's resistance and the end then came quickly as he claimed the 99th, and perhaps most valuable, title of his glittering career.
A memorable week ended with Italy's Sara Errani and Jasmine Paolini beating Russian neutral players Mirra Andreeva and Diana Shnaider 2-6 6-1 (10-7) to win the women's gold, adding to compatriot Lorenzo Musetti's men's singles bronze.
