The deal between Aramco and Malaysia's Petronas for the USD 27 billion project will be signed on Tuesday, the prime minister said.
"This is a huge investment and is very significant," said Najib.
The vast scheme under construction in the southern state of Johor near the Singapore border is known as the Refinery and Petrochemical Integrated Development Project.
It is being led by state energy firm Petronas, which said last month development is more than 50 per cent complete, with the project set to come online in 2019.
Najib told reporters: "The visit will cement and put our relationship on a strong trajectory."
He added the Saudi head of state was "satisfied and happy" the agreement would be signed.
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