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.
Meanwhile, Unilever appears to be firm on the rejection.
The Rotterdam-based group said the Kraft offer valued Unilever at around $143 billion, representing an 18 per cent premium over the share price at the close on Thursday.
"This fundamentally undervalues Unilever," it said in a statement, adding, it had "no merit, either financial or strategic, for Unilever's shareholders. Unilever does not see the basis for any further discussions."
But investors still massively bought into Unilever shares, which are quoted in London, pushing them over 12 per cent higher.
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".