South Korean shares closed at a record high on Thursday, with the country's primary index surpassing the 9,000-point level for the first time ever.

Investor sentiment was particularly buoyed by the US-Iran deal and the reopening of the Strait of Hormuz. In addition, the U.S. Federal Reserve's decision to keep its key rate steady in the range of 3.50%-3.75% at its latest rate-setting meeting also boosted sentiment.

The benchmark Korea Composite Stock Price Index, or Kospi, increased by 2.3%, or 199.6 points, to close at 9,063.84. The Kosdaq, however, decreased by 3%, or 31.03 points, to end at 1,000.93.

Thursday's gain marked the sixth straight day of gains, owing to optimism over the AI sector. Shares of market movers and chip stocks Samsung Electronics (KRX:005930) and SK Hynix (KRX:000660) rose nearly 5% and 7%, respectively.

U.S. President Donald Trump and Iranian President Masoud Pezeshkian signed the Iran document on Wednesday night, the White House told the BBC.

Key aspects of the deal comprise Iran's commitment not to develop a nuclear weapon, the restoration of shipping via the Strait of Hormuz with no transit fees for at least 60 days, the removal of U.S. sanctions, and an end to hostilities, including in Lebanon.

In corporate news, Samsung Biologics (KRX:207940) secured a biologics contract manufacturing organization (CMO) contract from an undisclosed US-based pharmaceutical company. The contract, valued at 835.7 billion won, is valid until Dec. 31, 2031.

Shares of Samsung Biologics added over 4% at market close.