"In a few months' time after the general elections, I will hand over the baton to a new prime minister," he said in opening remarks at his first full press conference in three years.
Singh, 81, had already hinted strongly at his intention to make way for leader-in-waiting Rahul Gandhi, the scion of the Gandhi dynasty which has dominated India's politics since independence.
He said that Congress would announce its prime ministerial candidate in due course.
"Rahul Gandhi has outstanding credentials. I hope our party will take that decision at an appropriate time," he added.
Polls show Congress are extremely unlikely to emerge as the winner in the world's biggest election due by May this year, with the opposition Bharatiya Janata Party making gains under hardline leader Narendra Modi.
"It would be disastrous for the country to have Narendra Modi as prime minister," he added in a strong attack on the Hindu nationalist ring-winger from western Gujarat state.
During his time as prime minister, Singh has seen his formerly stellar reputation built up through his work as a reforming finance minister in the 1990s tarnished by a string of corruption scandals and slowing economic growth.
He mounted a defence of his legacy, regretting high inflation, the graft scandals and weak growth in manufacturing output, but hailing his government's work for the rural poor and farmers.