The emphatic victory set the Jamaican showman off on his quest to complete the 'triple triple' -- 100m, 200m and 4x100m gold medals at three consecutive Olympics.
"Somebody said I can become immortal. Two more medals to go and I can sign off. Immortal," said Bolt.
Although well short of his world record, Bolt expressed satisfaction with his performance .
"It was brilliant. I didn't go so fast but I'm so happy I won. I told you guys I was going to do it," he added.
Minutes earlier, jaws dropped as van Niekerk timed a lightning 43.03sec in the 400m, breaking Johnson's 17-year-old record set in 1999.
"I have never seen anything like that," Johnson told the BBC. "It is amazing. That was a massacre by Van Niekerk. This young man has done something truly special."
The fireworks came after an emotional Andy Murray won his second straight men's singles tennis gold, and fellow Brit Justin Rose became the first Olympic golf champion in 112 years.