Valve, owner of online PC gaming platform Steam, and five other publishers have been fined a total of €7.8m (£6.9m) for restricting cross-border sales of PC video games.
So-called geo-blocking means games are locked, so that a cheaper licence intended for less well-off European countries cannot be used elsewhere.
That stopped gamers "shopping around" for the best deals, said EU Competition Commissioner Margrethe Vestager.
Valve said that it planned to appeal.
The other publishers fined were:
- Japan's Bandai Namco and Capcom
- French developer Focus Home
- German publisher Koch Media
- US-based ZeniMax
All five co-operated with the commission, so had their original fines reduced. But the commission said Valve did not co-operate, and it was fined more than €1.6m (£1.4m) with no reduction.
Valve denied this, telling the BBC that it has "co-operated extensively, providing all requested evidence and information to the commission".
"Approximately just 3% of all games using Steam (and none of Valve's own games) at the time were subject to the contested region locks," said the firm's spokesman Doug Lombardi.
"The elimination of region locks may also cause publishers to raise prices in less affluent regions," he added.
It added that it was being punished for declining "to admit that it broke the law, as the EC demanded".
Companies are not supposed to split up the single European market. But some members of the European Economic Area (EEA) have lower incomes than others, meaning that games are generally priced more cheaply for those countries.
Valve makes games such as the Half-Life franchise, but also owns Steam, one of the world's largest online PC gaming platforms, offering an estimated 44,000 games for sale.
Buyers can buy games directly from Steam, but the company also gives game publishers "activation keys" for its game downloads that can be bought on other sites.
These activation keys, which publishers agreed to region-block with Valve, were at the core of the investigation.
The publishers were found to have geo-blocked around 100 PC games from a variety of genres including sports, simulation and action.
According to the EU, Valve agreed bilateral deals with all the named publishers to issue Steam keys that prevented activation of certain games outside the Czech Republic, Poland, Hungary, Romania, Slovakia, Estonia, Latvia and Lithuania.
Ms Vestager said: "Today's sanctions against the geo-blocking practices of Valve and five PC video game publishers serve as a reminder that under EU competition laws, companies are prohibited from contractually restricting cross-border sales.
"Such practices deprive European consumers of the benefits of the EU digital single market, and of the opportunity to shop around for the most suitable offer in the EU."
Meanwhile, the five publishers formed deals with each other restricting cross-border sales of games, the EU found.