A US court has ordered Samsung to pay Apple $1.05bn (£665m) in damages for infringing intellectual property.
The jury decided several Samsung devices had infringed iPhone-maker Apple's software and design patents, but rejected counter-claims by Samsung.
Apple will now seek import bans on several of its rival's products. Samsung has said it will appeal.
Correspondents say the ruling is one of the most significant in a global battle over patents and intellectual property.
In recent weeks, a court in South Korea ruled that both technology firms had copied each other, while a British court threw out claims by the US company that Samsung had infringed its design rights.
But the year-long US case has involved some of the biggest damages claims, and is likely to shape the way patent licences are handled in the future.
Samsung promised to appeal against the decision describing it as "a loss for the American consumer".
"It will lead to fewer choices, less innovation, and potentially higher prices," the South Korean firm said.
The statement added that it was "unfortunate that patent law can be manipulated to give one company a monopoly over rectangles with rounded corners".
Apple, however, said it applauded the court "for finding Samsung's behaviour wilful and for sending a loud and clear message that stealing isn't right".
Apple said it intended to seek sales injunctions at a follow-up hearing on 20 September
The two firms account for more than half of global smartphone and tablet computer sales.
The nine-person jury at the federal court in San Jose, California had to consider 700 questions about each side's claim that its rival had infringed its intellectual property.
It deliberated for less than three days before coming to a unanimous decision, rejecting all of Samsung's claims and upholding five of Apple's allegations, including:
- Some of Samsung's handsets, including its Galaxy S 4G model, infringed Apple's design patents for the look of its iPhone including the system it uses to display text and icons
- All the disputed Samsung devices had copied Apple's "bounce-back response", which makes lists jump back as if yanked by a rubber band
- Several Samsung devices incorporated Apple's facility allowing users to zoom into text with a tap of a finger
Apple had wanted $2.5bn in damages. Samsung had sought $519m.
Michael Gartenburg, research director at Gartner, told the BBC it could be a good thing for consumers in the long run because it would force Apple's competitors to innovate.
"Anyone who was even thinking about borrowing a technology or design from Apple will think twice about it now," he said.
Other analysts point out that Apple could be the overall loser because the court case has helped boost Samsung's profile.
However, Christopher Marlett of investment bank MDB Capital Group said there was a "social cost" for Samsung.
"As a company, you don't want to be known as someone who steals from someone else," he said.
Apple remains one of the South Korean company's biggest customers buying computer chips and, reportedly, screens.
Sansung has already brought out a new generation of products that should avoid the patent issues.
But Apple said it still intended to seek sales injunctions at a follow-up hearing on 20 September.
It may also seek to use this ruling to block other devices powered by Google's Android software that it believes replicate elements of its user-interface, including current models by Samsung as well as other firms.