Zou, who won China's first Olympic boxing medal, a bronze, eight years ago and its first gold in front of home fans at the Beijing Games, added a second Olympic win to a career haul that also includes three world amateur championships.
The canny 31-year-old, who survived a couple of scares to reach the final, grinned all the way to the ring and was probably still smiling inside when he was slightly fortunate to be narrowly ahead after the first round.
Pongprayoon, at 32 the oldest among the men's 20 finalists, was cheered on by the majority of the crowd, including a noisy section of Thai fans, but their encouragement turned to boos after the second round when he was again unlucky to lose by a single point.
The Thai fighter continued to be a nuisance against his taller opponent in the final round, catching Zou with a big left but the now double Olympic champion, who was warned for holding, hung on to win 13-10.
Pongprayoon, who also received a warning in the final seconds of the bout, fell to the ground in tears and beat the canvass in frustration as a flag-waving Zou was booed.
Pongprayoon's cornerman even raised his fighter's hand in defiance to the delight of the crowd.
Losing semi-finalists Ireland's Paddy Barnes and David Ayrapetyan of Russia took bronze.
Meanwhile, classy Roniel Iglesias Sotolongo won Cuba's first Olympics boxing gold in eight years when he beat Ukraine's Denys Berinchyk in a battle which pitted the great amateur boxing nation against the rising force of the sport.
Sotolongo, one of the most clinical boxers at the Games who beat world light-welterweight champion Everton dos Santos Lopes of Brazil on his way to the final, caught the aggressive Ukrainian with one left upper cut too many to win 22-15.
"Today is the day which is the most important in my whole career," Sotolongo said. "Without my family I could not achieve this victory. They will be so proud, I dedicate this to them."
Sotolongo, one of four Cuban bronze medallists at the Beijing Games, took a tight first round 5-4 after both men came out aggressively, trading powerful right and left jabs, and the Cuban's left upper-cut proved the difference.
It was the same story in the second round with the 23-year-old Cuban proving too quick for Berinchyk who has sported one of the oddest haircuts at the Olympics, an eye-catching mostly shaved style with a lengthy piece of hair left on top.
The Ukrainian world championship silver medallist may have produced one of the tournament's best rounds in the semi-finals, scoring more points in the final three minutes than most have managed in nine, but there was little chance of a repeat on Saturday.
Sotolongo was just as relentless in the last round, delivering a series of stinging right jabs before beating his chest as the final bell rang and waving the Cuban flag in a victory lap of honour once the result was confirmed.
Losing semi-finalists in the light-welterweight class, Vincenzo Mangiacapre of Italy and Munkh-Erdene Uranchimeg of Mongolia, returned to the arena on Saturday to accept their bronze medals.
Japan's Ryota Murata denied Brazil a first Olympic boxing gold medal when he fought smart to win his country's first in the ring for 48 years by beating middleweight Esquiva Falcao Florentino.
Falcao, whose brother Yamaguchi won bronze in the light-heavyweights, appeared to serve notice to the rest of the middleweight division when he demolished Britain's Anthony Ogogo in the semi-final.
However university worker Murata, a silver medallist at last year's world amateur championships, gave the aggressive Falcao a lesson in how to score points, defending expertly and catching the Brazilian each time he came in close to win 14-13.
"I'm really happy but the result was really out of my control. All I could do was pray to God," said Murata, who easily beat Falcao in the semi-finals of the world championships.
"I work at a university and all my focus was on the boxing, so now I can concentrate on paying back all the teachers, students and families."
The 26-year-old Japanese countered superbly in the first round to open up a two point lead and although Falcao cut that back to a single point after the second round thanks to a couple of big lefts, Murata refused to panic.
When Falcao received a warning in the final round his fate was sealed and the Brazilian, whose first name Esquiva means "dodge" in English, nodded in agreement as the judges gave the bout to Murata by just a point.
Falcao, who flirted with a life of street crime before his father Touro Moreno, a big name in Brazilian boxing, intervened, remembered his humble roots after achieving South America's largest and most populous country's best boxing result.
"I'm very happy. I wanted a lot to achieve this moment. It's the most important day of my career," said the 22-year-old Brazilian, who wants to fight in his home Games in Rio de Janiero in four years time along with a third Falcao brother, Estiva.
"It is very important for my family, I come from a humble family and this is very important for all of them. There will be a huge celebration for me when I get back to Brazil."
Japan's last gold in the boxing ring was won by Takao Sakurai at his home 1964 Games in Tokyo while Brazil's only previous boxing medal before the three won in London came in the 1968 Games.
Ogogo and his fellow losing semi-finalist Abbos Atoev of Uzbekistan joined Murata and Falcao on the podium to accept their bronze medals.
Oleksandr Usyk won Ukraine's first boxing gold medal of the Games leaving Italy's Clemente Russo to pick up a second successive heavyweight silver medal.
Ukraine dominated last year's world championships, taking home four gold medals including one for Usyk, as they seek to muscle their way to the top of the amateur game.
Usyk, who at 190 centimetres (6 feet 3 inches) enjoyed a nine centimetre height advantage over the Italian, boxed cautiously for the first round and paid the price as the awkward Russo caught him with some swiping left hands.
Russo, one of the poster boys of the Olympic boxing governing body's new professional league, may have ridden his luck to reach Saturday's final but he showed far more guile than in previous bouts to chalk up a two point lead.
However the big Ukrainian opened up in the second round and showed more of the aggression that saw him thoroughly punish Bulgaria's Tervel Pulev in the semi-finals to peg back Russo's advantage and go into the final round level.
With both fighters out on their feet, Usyk, who like team-mate Berinchyk, sports one of the oddest haircuts at the Olympics, delivered some decisive big left hooks in the final minute to take the contest 14-11.
Usyk danced his now customary elaborate victory jig when the score was announced, adding an extra spin or two this time and he will likely be celebrating again on Sunday when team-mate and hot favourite Vasyl Lomachenko fights for his second Olympic gold in a row.
Losing semi-finalists Pulev and Teymur Mammadov of Azerbaijan took bronze.