The 21-year-old, who was subjected to gender test following her win at the 2009 Worlds, went through in two minutes 00.71 seconds.
Britain’s Lynsey Sharp, American Alysia Johnson Montano and Kenyan defending champion Pamela Jelimo also all comfortably qualified.
Sarah Attar, Saudi Arabia’s first woman track athlete, was last in her heat.
She was given a standing ovation at the Olympic Stadium as she went over the finishing line.
“Semenya ran quite well there, but [to] the winner Montano of the United States: this is how not to run an 800m race. Setting off quite quickly then slowing down is a silly tactic because it makes you more tired than you need to be. The women’s 800m is more about judgement than setting a pace.”
Sharp, 22, was chosen ahead of four other British athletes who had run faster qualifying times, but she said her controversial selection had not been weighing on her mind.
“I’ve said all along that I tried to block it out and not take it as added pressure,” said Sharp, who ran 2:01.41 to advance. “I’m in great shape.”
Semenya became global news when she was asked to take a gender test by athletics’ governing body IAAF in the aftermath of winning her world title.
Doubts were raised over her gender because of her muscular physique, running style and sudden improvement in times.
Semenya was suspended for 11 months by the IAAF but was cleared to return to the track in July 2010.
She won silver at last year’s World Championships in Daegu despite a back problem.
“It was a tactical race,” said Semenya, who carried South Africa’s flag at the opening ceremony in London, after her 800m heat. “I wanted the race to be a fast one.
“I have to run a sub-two (minutes) race to be a contender.”-www.bbc.co.uk