Shares in Kia, an affiliate of Hyundai Motor Co, jumped as much as 14.5% to hit its highest since 1997 at 102,000 won on Wednesday.
The rally came after South Korea's online news outlet DongA.com reported that Apple would invest $3.6 billion in Kia as they collaborate to produce Apple's electric vehicles in Kia's Georgia plant, without citing any sources. The report said the deal would be signed on Feb. 17.
Apple will aim to produce 100,000 vehicles annually by 2024 at the Kia plant, the report said, aiming to expand that annual capacity to 400,000 at a later stage.
Apple and Hyundai declined to comment when contacted by Reuters.
Kia shares surged nearly 20% on Jan. 20 after a media report said its parent Hyundai Motor Group had decided Kia would be in charge of proposed cooperation with Apple on electric cars.
Kia at the time said it was reviewing cooperation on self-driving electric cars with multiple foreign firms, without mentioning the report linking it to a project with Apple.
Reuters reported last week that Hyundai Motor Group has "tentatively decided" that it would want Kia to partner with Apple, citing a Hyundai insider.
Shares in Hyundai Motor were up 1.7%, while affiliates Hyundai Mobis Co Ltd and Hyundai Glovis Co Ltd rose 3.3% and 6.7% respectively as of 0319 GMT, outperforming the 0.1% gain in South Korea's broader stock market KOSPI.
(Reporting by Heekyong Yang; Editing by Ana Nicolaci da Costa)