TOKYO—A Japanese supercomputer built by Fujitsu Co. FJTSY -0.74% grabbed the title of world's best-performing machine from a Chinese competitor, returning Japan to the top of the computer arms race for the first time in seven years.

Installed at Japan's Institute of Physical and Chemical Research and also known as Riken, the Japanese government-funded "K Computer" performs more than eight quadrillion (8,000 trillion) calculations per second. K Computer is a play on the Japanese word "kei" for the number 10 quadrillion, which will be the number of calculations the machine is targeted to handle once it is completed in 2012.

In an era marked by China's growing technological and economic emergence, the return to the top of the supercomputer heap will be a source of pride for Japan only a few months after China overtook it as the world's second-biggest economy.

An employee assembles parts for a Fujitsu supercomputer in 2010. Fujitsu's "K Computer," which performs more than 8 quadrillion (8,000 trillion) calculations per second, was named the world's best-performing machine, the first time a Japanese computer has won the title in seven years. Bloomberg News

The Japanese machine is a major step up from existing supercomputers. It is more powerful than the next five fastest computer systems combined, and can perform three times as many calculations per second as the No. 2 supercomputer, designed by China's National University of Defense Technology, according to the "Top500," a compilation of the 500 most powerful computer systems in the world, determined by a group of academic and government researchers.

Investing more than 100 billion yen ($1.25 billion) in the K Computer project, the government aims to position Japan among the leaders for supercomputers, which can be used to tackle complex problems related to climate change and weather patterns. The project also aims to increase the competitiveness of Japan Inc. by providing a powerful computational tool to develop breakthroughs in drugs, materials and new technologies.

At a joint news conference with Fujitsu and Riken, the project's leaders said the goal was to find a wide range of real world applications for the K Computer. Researchers said the computer can be used to bolster the push for renewable energy by discovering the most efficient materials to convert the sun's rays to electricity, or protect people from natural disasters by predicting the impact from earthquakes and tsunami.

"We think this is one part of Japan's strength and we strive to benefit all of Japanese society with it," Ryoji Noyori, who is the president of Riken and a Nobel Prize winner for chemistry, said at a news conference.

The K Computer is packed with computing muscle. It stitches together 68,544 processors, each equipped with eight cores for a total 548,352 electronic brains. At full capacity, it aims to have 640,000 electronic brains. Fujitsu said this would provide the machine with enough horsepower to slash the time required to run a simulation of a beating human heart reacting to new medicine to two days from two years.

The system is housed in 672 refrigerator-sized computer racks at Riken's Advanced Institute for Computational Science in Kobe in western Japan. The machine uses 9.89 megawatts of power, roughly enough electricity to power more than 9,000 homes per year. However, it is energy-efficient compared with the average energy draw of its top 10 rival supercomputers: It can perform nearly twice as many calculations per watt of energy.

Japan started building supercomputers in the mid-1980s, a time when the country's electronics companies already dominated many technology segments. During those years, the Japanese government and its companies invested heavily in supercomputing. While Japan never overtook the U.S. in the overall number of high-end supercomputers, its fastest machines often jockeyed with U.S. systems for the top spot.

Japan last held the top spot in 2004, with NEC Corp.'s 6701 0.49% Earth Simulator. In a sign of the rates of improvement for supercomputers, the K Computer is more than 200 times more powerful than the Earth Simulator. By 2018, Japan, the U.S. and China are targeting the development of supercomputer capable of doing 1 quintillion (1 million trillion) calculations per second.

The supercomputer was almost scuttled when a government panel urged the freezing of the project's budget in 2009, citing it as an example of wasteful government spending and criticizing the push to become No. 1 as unnecessary. It survived after a group of Nobel prize winners appealed to then-Prime Minister Yukio Hatoyama.

There are five U.S. supercomputers in the top 10 ranking, including the third-ranked system called Jaguar at Oak Ridge National Laboratory in Oak Ridge, Tenn. The rest of the top 10 include two machines from China, two from Japan and one from France.

The Top500 list is compiled by researchers at the University of Mannheim in Germany, the University of Tennessee and the National Energy Research Scientific Computing Center, which is affiliated with Lawrence Berkeley National Laboratory in California.

The twice-a-year rankings fluctuate greatly. China held the top position in November 2010 and a U.S. machine was No. 1 in June 2010. Japan's hold on the top spot may be short-lived: International Business Machines Corp. is building a system to break the 10 quadrillion calculations per second barrier at the National Center for Supercomputing Applications at University of Illinois.

Write to Daisuke Wakabayashi at Daisuke.Wakabayashi@wsj.com