Kraft Heinz is the fifth-biggest food and beverage company in the world and the third-biggest in North America.
Despite being turned down, the US company would continue working towards a deal to combine both groups, it said in a statement filed with the London Stock Exchange.
"Kraft Heinz confirms that it has made a comprehensive proposal to Unilever about combining the two groups to create a leading consumer goods company with a mission of long-term growth and sustainable living," it said.
"While Unilever has declined the proposal, we look forward to working to reach agreement on the terms of a transaction," Kraft Heinz said, adding however there was "no certainty" that there would be a further proposal.
Kraft Heinz is the fifth-biggest food and beverage company in the world and the third-biggest in North America. It this week reported sales of $6.86 billion for the fourth quarter ending December 31.
Unilever, which had sales of 52.7 billion euros ($56.1 billion) in 2016, owns more than 400 brands including Lipton, Knorr, Dove and Omo.
Its strategic focus is on sustainable growth which it calls "the only acceptable model for our business".