Koizumi told supporters that his son, Shinjiro, 27, will run for his seat, the Financial Times reported. Koizumi was prime minister for 5 ½ years, one of the longest terms since World War II in a country where the Liberal Democratic Party has tended to remain in power while its officials come and go.
He took office while Japan was in an economic slump and presided over its recovery.
The country has had three prime ministers since Koizumi stepped down in 2006. His decision to leave parliament was a surprise to members of his party even though Koizumi had stayed out of the limelight in the past two years.
Prime Minister Taro Aso must call a general election by next September, although he is expected to do so earlier.