Japanese puts in mammoth free skate to beat Russia's Andrei Mozalev and Daniil Samsonov
The 16-year-old was in imperious form, scoring a cool 166.41 points – a new personal best by nearly six points – for a total of 239.17.
Mozalev, who was fifth in the Russian senior nationals, and 14-year-old fellow Russian Daniil Samsonov both suffered falls to fall back from their top two positions after the short program.
Samsonov took bronze with a score of 215.21, while Mozalev won silver on 237.94 points.
Kagiyama stamped his mark on proceedings at the Lausanne Skating Arena to clinch the gold.
He opened with a beautiful quadruple toe loop, laying down a strong early marker. The toe loop seemed to be his go-to jump, as he did it again in a quadruple toe, double toe combination and a triple flip, triple toe sequence.
Kagiyama also threw in two triple Axels, one in combination, for good measure.
His only mistake appeared to be on his first Axel, in which he turned a step out into a half-loop Euler for a follow-up triple Salchow in combination.
Mozalev, meanwhile, will have to be content with silver despite a solid skate of his own.
The 16-year-old Russian fell on a triple Axel and seemed to have an odd choice of a music mix, but still impressed the judges for a total segment score of 158.22.
Skating first of the top three, young Samsonov did not get off to an auspicious start.
He popped his opening Axel, although bounced back with a massive quadruple Lutz that had the crowd in awe.
He then fell on a triple Axel attempt and popped a later triple Lutz down to a double, but rebounded again with a big triple flip, triple toe loop combination.
It was not the Russian's best day on the ice, although his score was enough to guarantee him a medal.