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Buatsi is guaranteed at least bronze after winning by unanimous decision.
The 23-year-old will fight Kazakhstan's Adilbek Niyazymbetov on Tuesday for a place in Thursday's final.
Savannah Marshall remains in contention for a medal after winning her women's middleweight first-round bout.
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The 25-year-old, who won Britain's first-ever women's boxing world title in 2012, defeated Sweden's Anna Laurell on points.
However, Pat McCormack lost his light-heavyweight fight by split decision to Cuba's number two seed Yasnier Toledo.
In Rio to watch the next generation of boxers is retired former five-division world champion Floyd Mayweather Jr.
He has spoken to Buatsi, who says the 39-year-old told him he has what it takes to go to the top.
"He said to me he had heard about me before I got here and that I was a great fighter, a future world champion," said Buatsi.
"It was good to hear that from someone like Mayweather, who I think is the greatest boxer of our generation.
"I want to dominate in every way I can, and I want more than bronze. I will give it everything that I've got to make sure I get a gold."
Joshua Buatsi will become the first British boxer to win an Olympic medal in Rio after beating Algeria's Abdelhafid Benchabla to reach the light-heavyweight semi-finals.