Since the deal was announced this week, Trump has said he would not return to Twitter.
In Thursday's message, called a "truth" on the app, Trump wrote, "I'M BACK! #COVFEFE", referencing a typo on a Twitter message he sent while president that complained about the press and was widely memed.
Trump's silence on his own app since the launch of Truth Social on Apple Inc's (AAPL.O) app store on 21 February has raised questions about its long term viability.
Ahead of its launch, his son Donald Trump Jr. tweeted a screenshot of his father’s Truth Social account with one “truth” that he posted on Feb. 14, verified at the account of @reaDonaldTrump, with the message: “Get Ready! Your favorite President will see you soon!”
Trump Media & Technology Group (TMTG), Truth Social's parent company, is planning to go public through a merger with blank-check firm Digital World Acquisition Corp (DWAC.O), shares of which were up 7.7 per cent pre-market Friday.
The deal is under scrutiny by the US Securities and Exchange Commission and is likely months away from being finalised.