"The only reason they did it is because she's running for the office of president. They want to look tough. They want to look good," he said at the final presidential debate in Las Vegas.
Thousands of Iraqi and Kurdish forces are aiming to take back the city after more than two years under the terror group's control. It is considered a crucial operation in reducing ISIS positions in the Middle East.
Trump did not offer evidence for his assertion that the offensive was designed to help Clinton. Trump claimed that ISIS only took Mosul because U.S. forces in the region left under President Barack Obama's administration.
"I'm just amazed that he seems to think that the Iraqi government and our allies and everybody else launched the attack on Mosul to help me in this election," Clinton said. "But that's how Donald thinks, you know, he's always looking for some conspiracy."
Trump also contended that Iran would be the big winner if U.S.-backed forces did take Mosul.