King Bhumibol Adulyadej endorsed the election date on Thursday, ending months of speculation over when polls would be held.
The announcement of the king's approval by Rongphol Chareonphanthu, the cabinet secretary, was the latest move aimed at easing uncertainties and ending the political limbo that has led to rumours of a military coup.
Thailand's supreme military commander, General Ruengroj Mahasaranond, reiterated on Friday that the armed forces would not step into politics.
The problems began in January, when the family of Thaksin Shinawatra, the prime minister, earned $1.9 billion in a tax-free stock sale that prompted months of street protests and caused Thaksin to call elections three years early on April 2.
His Thai Rak Thai party won the vote, but the victory was undermined by an opposition boycott, leading the Constitutional Court to invalidate the result in May.
Questions remained over if and when polls could be scheduled after a raft of lawsuits regarding the April election were filed.
Analysts fear street protests
against Thaksin could resume
In the most serious case, the Constitutional Court is considering charges of vote fraud against Thaksin's ruling Thai Rak Thai and the main opposition Democrat Party.Some political analysts say the new election, run by an Election Commission accused of favouring Thai Rak Thai, could create as much chaos as the vote in April if the current commissioners are still in place.
"We will see the reincarnation of the April 2 poll results," Thitinan Pongsudhirak of Bangkok's Chulalongkorn University said.
A network of anti-Thaksin protesters said street rallies aimed at ousting him would resume, but did not set a date.
Thaksin, whose support in the countryside has already won him two landslide election victories, stepped aside in April after it became clear that the election would not produce a valid parliament.
But he returned just two months later to lead a caretaker government that has been unable to make major policy decisions, including plans to spend billions of dollars on infrastructure projects.
The main opposition Democrat Party said it would put up candidates in October and not repeat its April boycott.
"We are getting ready to declare a 'People's Agenda' in the election campaign," a spokesman for the party, Ong-Ard Klampaiboon, said.