The country's biggest drugmaker said it had agreed to provide capacity required for the manufacture of J&J's vaccine candidate, which is still in clinical trials, at its Port Elizabeth facility.
The agreement with J&J will see Aspen perform formulation, filling and secondary packaging of the vaccine for the U.S. firm, the company said.
Aspen's shares rose more than 4% on the Johannesburg Stock Exchange on the announcement.
The company said it had invested more than 3 billion rand ($184.2 million) in the Port Elizabeth facility.
"The production area where it is intended that the vaccine candidate will be manufactured has capacity to produce more than 300 million doses per annum," it added.
South Africa has recorded more than 726,000 coronavirus infections, the highest number on the African continent, and more than 19,400 deaths.
It is hosting at least four clinical trials of potential COVID-19 vaccines, including the J&J candidate.
The agreement between Aspen and J&J is subject to the completion of technology transfer activities and finalisation of certain commercial terms.
Last month South Africa's Biovac Institute - a joint venture between the government and private sector - said it was in talks to produce some of the vaccines the country needs to protect itself against COVID-19.
($1 = 16.2888 rand)
(Reporting by Tanisha Heiberg; Editing by Alexander Winning and Emelia Sithole-Matarise)
By Tanisha Heiberg