"People of India join me in sending our sincerest condolences to the Thatcher family, the government and people of the United Kingdom," he said in his condolence message.
Singh said Thatcher was "a transformative figure under whom the United Kingdom registered important progress" in the national and international arenas.
Thatcher, Britain's first woman Prime Minister, also known as the 'Iron Lady', passed away today following a stroke at the age of 87.