The 22-year-old, whose previous deal was set to expire in June, expressed his delight with the fresh terms.
"Obviously I am delighted at the three-year deal I signed with Celtic, who are one of the biggest clubs in the country,” Manzini said on Phunya Sele Sele's official website.
"The supporters are incredible, the management and club very professional and I am looking very much forward to the challenge."
Celtic CEO Khumbulani Konco believes Manzini can develop into one of the top marksmen in the country.
"We are very pleased to have signed Lerato Manzini for a further three years as the club really believes he is one of the most promising strikers in the League," said Konco.
"He has been a bit unlucky with injuries but he has a great work ethic and fought his way back to fitness in a very professional manner.
"He has also proved to be a goal-scorer, which is something that is sometimes lacking in this country, and we believe he can be one of the best."