Hershey stock was on track for its worst daily performance since September 2002, when the shares fell 11.9 percent. That decline was triggered when the Hershey Trust, the company's controlling shareholder, told the chocolate maker to terminate its proposed deal with Wrigley.
Shares of Mondelez, meanwhile, gained nearly 4 percent on the day.
Mondelez CEO Irene Rosenfeld said in a statement Monday that after additional discussions, the company decided "there is no actionable path forward toward an agreement."
Rosenfeld faced staunch opposition from the Hershey Trust, a $12 billion charity created by the company's founder a century ago. The trust owns 34 percent of the company's shares and controls about 80 percent of the vote.
If it had agreed to merge with Mondelez, Hershey stood to lose its U.S. license for Kit Kat, which is produced globally by Nestle.