He said the platform would "stand up to the tyranny of big tech", accusing them of silencing opposing voices in the US.
Trump Media & Technology Group (TMTG), which he chairs, also intends to launch a subscription video-on-demand service.
Mr Trump was banned or suspended from social networks like Twitter and Facebook after crowds of his supporters stormed the US Capitol in January.
He and his advisers have since hinted that they were planning to create a rival social media site.
Earlier this year, he launched From the Desk of Donald J Trump, which was often referred to as a blog.
The website was permanently shut down less than a month after it launched. His senior aide Jason Miller said it was "just auxiliary to the broader efforts we have and are working on".
An early version of his latest venture, TRUTH Social, will be open to invited guests next month, and will have a "nationwide rollout" within the first three months of 2022, according to a statement by TMTG.
"We live in a world where the Taliban has a huge presence on Twitter, yet your favourite American President has been silenced," wrote Mr Trump.
"Everyone asks me why doesn't someone stand up to Big Tech? Well, we will be soon!" he added.