Athletics Kenya, the governing athletic body, said the country lost a great athlete. The association said it had pinned its hopes on Wanjiru to win another medal at the London Olympics in 2012.
"He's left a gap that will be hard to fill," said David Okeyo, the secretary-general of Athletics Kenya.
The International Olympic Committee praised Wanjiru as an accomplished runner who will be remembered for winning his country's first Olympic gold medal in the marathon and setting a new Olympic record in the process.
"Our thoughts are with his family and friends," the IOC said in a written statement sent to CNN.
At 21, Wanjiru was the youngest gold medal winner in the marathon at an Olympics since 1932.
After his Beijing triumph, Wanjiru has set about dominating the marathon distance, claiming a number of victories in races.
It helped him be crowned the men's champion in the 2009 and 2010 world marathon majors series.
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