"It is my profound pleasure to convey, on behalf of the royal government and people of Cambodia, our warmest and heartfelt congratulations to you on the auspicious occasion of your appointment as the prime minister of India," Xinhua quoted Hun Sen as saying in a congratulatory message.
He expressed his confidence that under Modi's leadership, the people of India would enjoy greater prosperity and happiness in the years to come.
"I look forward to working closely with you to further deepen and expand the relation of close friendship and cooperation between Cambodia and India for the mutual benefits of our two countries and peoples," the prime minister said.
The Bharatiya Janata Party (BJP), led by Modi, won a landslide victory in India's general election.
Modi will be sworn in as India's 14th prime minister May 26.