Shares in Samsung Electronics opened 6.75 per cent lower on Monday after a US court fined the South Korean firm $US1.05 billion for breaching Apple's patents.
After 15 minutes of trading the shares had fallen by 6.81 per cent to 1,187,000 won.
A jury in San Jose, California ruled on Friday that Samsung infringed six Apple patents for smartphones or tablet PCs.
Analysts say the judge in the case could triple the damages because jurors found Samsung acted "wilfully". They rejected the Korean firm's counterclaims against Apple.
Samsung says it will contest the verdict.
"The context of the loss in this case is a lot worse than had been expected," including the amount of damages Samsung is now facing, said Hi Investment and Securities analyst Song Myung-Sup.
If the ruling leads to a US sales ban on the Samsung products subject to dispute, it will start eating away the company's revenue from the fourth quarter, Song said.
Analysts say a key point will be whether the patents battle spreads to Samsung Electronics' flagship Galaxy S III phone. The products ruled to have infringed Apple's patents are all older models.