Mr Singh said it was a "legitimate desire" of Indians living overseas to have a say in who governs the country.
He was speaking at the annual meeting of NRIs, held in the capital, Delhi. The meeting is attended by 1,500 delegates from more than 40 countries.
Four years ago, the government launched an overseas citizenship scheme for the 25 million-strong Indian diaspora.
It allows people of Indian origin who are not Indian nationals most of the same rights as Indian nationals.
However, they are not allowed to vote in Indian elections.
"I recognise the legitimate desire of Indians living abroad to exercise their franchise and to have a say in who governs India," the Times of India website quoted the prime minister as saying.
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