SEOUL—South Korea's government is wiping out rules for Web browsing that trapped the country's Internet users with 1990s-era security technology and created a de facto monopoly for Microsoft Corp.'s Internet Explorer here.

The South Korean rules had long stood in contrast to efforts by other governments around the world that have tried to break Microsoft's grip on the Internet browser market. For South Koreans, they made Internet transactions a nuisance by requiring that users download plug-in programs,...