Baron Waqa has been president since 2013 and his support was critical to hosting Australia’s asylum seeker detention facility on the island.
Waqa, 59, had failed to secure enough votes to hold on to his seat in Saturday’s election, according to preliminary returns to the Nauru Electoral Commission, Australia’s national broadcaster ABC reported.
A final announcement of the election result is expected on Monday.
Since 2001, Australia has placed asylum seekers caught trying to reach Australia by boat in the Nauru detention facility while their refugee status was processed.
Nauru made about a third of its national income from the arrangement.
Nauru is the world’s smallest republic, with just 7,000 people eligible to vote for the 19-member parliament that chooses the president.
It is the world’s third-smallest nation behind Vatican City and Monaco.
