SIMPEC, a subsidiary of ASX-listed WestStar Industrial, will design and construct a remote draft survey system for the Cape Lambert port.
The work will kick off this month and be completed by the second quarter of 2019.
SIMPEC managing director Mark Dimasi said the win was a testament to the team's hard work during the six-month projects tender phase.
"What a commendable achievement to secure a project directly with Rio Tinto, a global tier-one company," he said.
It continues a strong run for the contractor.
Late last month, SIMPEC won a $1.7 million contract from Iluka Resources for a flocculant plant at Cataby, then this month it was awarded a $4 million electrical and communications contract at Rio's West Angelas mine, and an $8 million piping contract for Tianqi Lithium's Kwinana plant.
The company now has an order book of around $30 million and has tendered for more than $150 million of work over the past year.
"To deliver an order book of $30 million only halfway through the year is a tremendous result for SIMPEC and the whole WestStar group," WestStar director Philip Re said.
"SIMPEC is building great momentum and traction in the market and we look forward to the award of further contracts."
Last week the company secured a working capital facility of up to $1 million.
Shares in WestStar were unchanged at 1.5c, valuing the company at $7.4 million.