The companies gave no details on how many vaccines will be made as part of the "fill and finish" deal that will run through 2022. This type of a contract involves putting vaccines into vials or syringes, sealing them and packaging them up for shipping, but not making the vaccine itself.
It was also unclear if the Moderna deal would mean faster access to more doses for South Korea, which has managed to give just 7.4% of its 52 million population at least one dose due to supply snags.
U.S. vaccine maker Novavax also reaffirmed its partnership with South Korean firm SK bioscience Co Ltd to expand vaccine production. SK shares rose 1.9% compared to a 0.3% fall in the wider market.
Samsung BioLogics pared gains to close down 0.4%, while SK bioscience shares closed up 0.9% compared with a 0.4% fall in the wider market.
The announcements came after South Korean President Moon Jae-in visited U.S. President Joe Biden, and the two leaders promised to collaborate on vaccines.
Moon had hoped to return home with a "swap" deal with Washington to hasten access to U.S.-made shots in return for domestically producing more doses later, but secured vaccines for only 550,000 South Korean soldiers.
South Korea already had a deal with Moderna to procure 40 million doses of its vaccine, but no timeline had been agreed.
The first batch of 55,000 Moderna doses will arrive on May 31, the Korea Disease Control and Prevention Agency said on Monday, adding that the drug safety ministry will decide later who will get those doses.
Moon, whose approval ratings have slipped over his handling of the pandemic, called the summit a success, saying the country's vaccine partnership with the United States will help make it a "vaccine hub."
"Best ever visit and best ever meeting," he tweeted.
"The announcement that the U.S. would send vaccines to us, in addition to 'Vaccine Partnership', was literally a surprise."
But South Korea's opposition denounced Moon for "paying cash, but returning with just a promissory note".
"How did we become a country that is moved to tears with vaccine aid for just 550,000 soldiers from the United States?" Daegu Mayor Kwon Young-jin said on Facebook.
(Reporting by Joyce Lee, Sangmi Cha; Editing by Sayantani Ghosh and Jacqueline Wong)
By Sangmi Cha and Joyce Lee