The documentary is from Oscar-winning filmmaker Laura Poitras.
In a statement, the network announced it would partner with distributor Neon on a theatrical release, before premiering the film on television in a few months.
The movie was filmed over six years, and tool in the 2016 US presidential election.
The documentary claims to take viewers closer than any previous film crew into Assange's inner circle.
"With unprecedented success, Poitras gives us the WikiLeaks story from the inside, allowing viewers to understand our current era of massive risks, headline-grabbing news, and the revolutionary impact of the internet on global politics," Showtime said in a statement.
Showtime described 'Risk' as a portrait of power, principles, betrayal and sacrifice when the stakes could not be any higher.
According to Showtime, the movie is a first-person geopolitical thriller, told from the perspective of a filmmaker invested in the worlds of state surveillance, and the cyberpunk movement.
Poitra's profile of the WikiLeaks founder is a follow-up to her Academy Award-winning 'Citizenfour', about leaker Edward Snowden and the NSA spying scandal.
"It's an exciting time to be working with Showtime and Neon," the 53-year-old enthused.
Assange has been holed up in the Ecuadorian embassy in London since 2012, where he took refuge to avoid being sent to Sweden.
The 45-year-old faces rape allegations in Sweden.
He fears that Sweden would decide to extradite him to the United States, over WikiLeaks' leaking of diplomatic cables and other classified material.