Ms Tsai has secured a second term as President.
“You are a great friend of Nauru & the Pacific region & we look forward to continuing to stand with Taiwan & stand up for democracy across the world,” the Nauru Government tweeted on its Twitter account.
In response the Ministry of Foreign Affairs of Taiwan thanked Nauru for the warm congratulatory message.
BBC reports the election is dominated by the island's relationship with China.
Ms Tsai secured just over 57% of the ballot - a record 8.2m votes - well ahead of her rival Han Kuo-yu.
Ms Tsai opposes closer ties with China, with Mr Han suggesting they would bring economic benefits.
In her victory speech, Ms Tsai told China to abandon its threat to take back the island by force.