The Dewsbury-based fighter defeated Mohammed Uddin from Lansbury ABC to claim the 60KG crown.
Last year, Hargreaves, nicknamed ‘Baby Canelo’ by his coaches and club-mates, was beaten in the national final.
But the 23-year-old has bounced back 11 months on to claim the 60KG National title.
“It has been a long time coming really,” said Hargreaves.
“I lost in the final last year, it has been a challenge to get back into the gym and train.
“To get so far again, I knew I had to win this time.”
Hargreaves trains up to four times a week at the Purge Boxing Academy in Alexandra Mills, Batley.
But when he is not in the boxing gym, he is still undergoing gruelling training sessions with his brother and former boxer, Ben Hargreaves.
“Losing last year, it motivated him (Josh) a lot more,” said brother Ben.
“It would’ve broken a lot of people (losing in the final) but it made him a lot stronger.
“He has never stopped, he is in the gym three to four days a week and when he is out of the gym I train him.
“We do sprints, jogging, circuits, so he doesn’t really give himself a day off, it has been hard but it is worth it.”
Josh continued: “It is hard; the dieting and the training.
“Not being able to go out, even when people are going out for a meal, I have to sit out.
“It is hard work.”
Hargreaves’ opponent in Saturday’s final held a 12-1 record from his previous 13 fights, which included six stoppages.
Hargreaves’ victory takes his record to an impressive 10-1 with his only defeat coming in last year’s national final.
The Dewsbury fighter is predominantly trained by Purge owner and head coach Mohammed Amin Patel but also receives guidance from Purge coaches Umar Arif, Mohammad Nadeem and Julian McGowan.
McGowan, who coached Dewsbury’s Gary Sykes to a British title victory, said that Saturday’s bout was a brutal contest but that the right man won.
“It was a strong opponent who he was fighting and when we saw him in the flesh, you just knew that it would be a hard fight,” said McGowan.
“He had stopped several good opponents on his way to the final – his record spoke for itself.
“It was always going to be a hard fight, he matched Josh in the first round in terms of ferocity and power.
“But Josh has an incredible engine and was a little bit more skilled.
“Josh was blocking, catching a lot of shots on the gloves and countering.
“It was probably the hardest amateur fight that I have seen for a long, long time.
“It reminded me of a few early Gary Sykes fights, it was that kind of high pace and high tempo and the right man won.”
The referee was forced to deliver two standing eight counts in the final round as Hargreaves showed his intelligence and power to overcome Uddin’s strong punching.
McGowan continued: “Josh got to the national finals last year with almost zero experience and he was still disappointed at losing in the final last year.
“But this year you got this sense that he was going to go out there an do whatever he needed to do to get the win.
“Mo Amin has come on an incredible journey as well, to have his third national champion in 12 months is incredible.
“There are several others involved, we are all a team and we all work hard.”
Hargreaves will enjoy a well-earned rest before returning to training and setting his sights on the Yorkshire belt early next year.
The Dewsbury boxer will then aim to enter the British Amateur Boxing Association ranks in March. Hargreaves’ also has ambitions to turn professional within the next 18 to 24 months.
The Purge Boxing Academy, based at Alexandra Mills in Batley are looking to build a strong stable of fighters following their success over the last 12 months.
McGowan is a professionally licensed coach who, along with several other coaches, holds training sessions several nights a week.