Zhang, the 24-year-old world champion and pre-tournament favourite, on Thursday won 4-1 over his compatriot Wang Hao, who has now lost three successive Olympics singles finals.
After winning Zhang leapt over the court surround to kiss the podium, before draping himself in a Chinese flag in front of a raucous near capacity 6,000 seat crowd, which included Prince Philip, Duke of Edinburgh, and his son Prince Edward.
China have now taken 22 out of 26 Olympic golds on offer in table tennis after Li Xiaoxia won the women's singles gold in another all-Chinese final on Wednesday.
"There isn't any secret. The Chinese and Asian people, their physical training is stringent and strict, but others can also achieve great success," Zhang said in his victory press conference, through a translator.
Asked about his victory dash, he said: "Of course it was spontaneous. If you planned everything you can't play well."
Zhang saw off Wang's challenge thanks to his flashing top-spin forehands, succeeding where women's world number one Ding Ning slipped up on Wednesday.
Germany's Dimitrij Ovtcharov, seeded 8th, took bronze after winning the third-place match 4-2 against Taiwan's Chuang Chih-Yuan.
He was the first European to get on the men's singles podium since Swede Jan-Ove Waldner won silver in Sydney 2000.
Zhang, born six months before table tennis was introduced as an Olympic sport in Seoul 1988, is only the fourth man to win the grand slam - Olympic gold after the 2011 world championships and World Cup. In all three finals he has beaten Wang Hao.
Zhang said his attention is now on winning a second gold in the team competition, which starts on Friday.
"I'm not too excited because we need to think about the team competition, that's the first goal. The team is representing the whole country, so I need to do everything to prepare myself for that," he said.
The final match started with nervy, short early duels and came to life at the end of the first set, edged by Zhang 18-16 after seven deuces.
From there it was mostly plain sailing for the favourite. Cheers from Chinese supporters picked up when Wang won the fourth set and took an early lead in the fifth, but Zhang snuffed out the fightback and edged that set.
Zhang, a big fan of Portuguese and Real Madrid soccer star Cristiano Ronaldo, was accused of arrogance at home after ripping off his shirt in celebration following his first world singles title in Rotterdam last year.
He now teams up with the 2008 Olympic singles champion Ma Lin and Wang in the team event, where they will be red-hot favourites to add another gold on Aug. 8.
Ovtcharov, who lost 4-1 to Zhang in the semi-final, said he was "the happiest person in the world" after his bronze.
"It would be great if there was a special recipe how you can beat the Chinese. But they practice very strong and are very well prepared physically and mentally, especially for the big tournaments," Ovtcharov said.