Greece retained their International Swimming Federation World Men's Junior Water Polo Championship title in Kuwait City, as the host nation were disqualified from the competition.
Greece defeated Serbia 6-4 at the Al-Nasar Sport Club in Kuwait's capital.
They went 2-0 up in the first quarter and were winning 3-1 at halftime before a scoreless third quarter.
An action-packed final period saw Serbia bring the score to 5-4 before Konstantinos Kakaris secured the victory for Greece just two minutes from time.
Goalkeeper Nikolaos Mitrakis, who was named the best in his position of the tournament, blocked many Serbian attacks to help his team to gold.
"It was indescribable going back to back," said Greek captain Alexandros Papanastasiou.
"It was hard work, not luck.
"The team proved that if it could do it once, it could do it again.
"I wish to thank my team, my coaches for making this possible."
It was the third title for Greece, having won in the Turkish capital of Istanbul in 2001 and in the Serbian capital of Belgrade in 2017.
Italy beat Croatia 9-6 for the bronze medal in a sour clash, where two Croatian officals and two Croatian players were red-carded.
Spain then secured fifth place with a 14-8 victory over Montenegro, while the United States kept Japan at bay 19-14 for seventh position.
Iran and Kuwait were disqualified from the tournament after their clash was abandoned in the third period for "illegal entering of substitute players into the field of play" and will not take a position in the final placings.