"I announce today the start of the heroic operations to free you from the terror and the oppression of Daesh," he said in a speech on state TV, using an Arabic acronym for Islamic State.
"We will meet soon on the ground of Mosul to celebrate liberation and your salvation," he said, surrounded by the armed forces' top commanders.
The assault on Mosul is backed by the US-led coalition and could be one of the biggest military operations in Iraq since the 2003 US-led invasion that toppled Saddam Hussein.
"We are proud to stand with you in this historic operation," Brett McGurk, US envoy to the coalition against Islamic State, said on Twitter at the start of the Mosul offensive.
Mosul is the last major stronghold of the ultra-hardline Sunni group in Iraq. Islamic State fighters are dug in, and they have forced civilians to stay in harm's way during previous battles.