His account was "'permanently" suspended after the Capitol riot in January 2021 but was reactivated after Elon Musk bought Twitter last year.
However the former president had not posted on his account until now.
Mr Trump's first post was of his mugshot, with the words "Never Surrender" and a campaign website link.
Mr Musk later reposted it with the comment "Next-Level".
In the wake of the 6 January 2021 riot in Washington DC, Twitter said in a statement: "We have permanently suspended the account due to the risk of further incitement of violence".
Since then Mr Trump has been posting on Truth Social, his own Twitter-like social media platform. However, it is not widely used.
Before his suspension, Twitter was the main platform used by Mr Trump to communicate with his followers.
After he was elected, he credited it as a key reason for his successful run for the presidency.
In 2017, he said: "Twitter is a wonderful thing for me, because I get the word out… I might not be here talking to you right now as president if I didn't have an honest way of getting the word out."
Some political analysts believe Mr Trump being off Twitter has helped him politically - keeping him out of the public eye while he regrouped.
Others though believe he needs to post on a social media channel with far greater reach than Truth Social - and X could be that platform.
With Mr Musk's acquisition of the company, some thought Mr Trump would come back to the platform sooner.
He had resisted doing so for months. It is still unclear whether he has returned to X for good or if this is a one-off.
However, Mr Musk will be celebrating. The former president is a huge driver of traffic and would potentially bring people to a site that has struggled to attract enough advertising revenue to cover its costs.
In less than three hours, Mr Trump's original post clocked up more than 36 million views and 530,000 likes.