"This happened because of you. Thank you," he tweeted to his 22 million followers.
"Four more years," he added straight afterwards, posting a photo of himself hugging First Lady Michelle Obama.
According to the Telegraph, the post, as of 8.20am local time, had 507,745 retweets and had been favourited 173,028 times.
This made it the most popular tweet ever, topping a message from singer Justin Bieber.
The same picture of a happy, serene-looking Obama hugging his wife appeared on the president's Facebook account - and was shared tens of thousands of times by some of his 32 million fans, the report said.
According to Twitter, the election has become the most tweeted about event in US political history, with some 31 million poll-related posts fired out throughout day.