The 32-year-old, a silver medallist in the same event at Beijing 2008, went one better this time by narrowly holding off the charge of Luca Tesconi.
Jin held a 4.4 lead at the halfway stage of the final but, after a flying start, suddenly lost his range while his Italian rival closed the gap to 1.3.
But with his last effort, Jin scored a near-perfect 10.8 to seal gold and leave Tesconi with silver.
Andrija Zlatic of Serbia took the bronze medal with reigning Olympic champion Pang Wei of China fourth.
Jin said: 'I am very happy because it is a first gold medal for Korea, and personally it is very meaningful.
'I think getting off to a good start is important, and I hope this medal will be a spur and inspire Team Korea to more success at these Games.'