"I call upon all Bangladeshis to remain united and face any attempt by anyone to thwart the election," Hasina said, after meeting party leaders at the airport.
"There is no alternative to an election at this stage."
Hasina was in the US on medical parole after Bangladesh's army-backed interim government allowed her to leave for medical treatment.
She had been detained in June 2007 when the interim government mounted a crackdown on corruption.
She spent a year in detention - and still faces charges of corruption, which she denies - before authorities allowed her to travel abroad.
Her Awami League party, which ruled Bangladesh for five years until 2001, is a front-runner in the December polls - the first elections in Bangladesh in seven years.
The party had asked the government to provide maximum security for Hasina on her return, as she has been a target for armed groups in the past.
She survived a grenade attack at a Dhaka rally she addressed in 2004 that killed 23 people and wounded more than 150.