Komatsu will retrofit 29 haul trucks with Autonomous Haulage System (AHS) starting next year, with the Brockman 4 operation to run entirely in AHS mode by mid-2019.
A further 19 Caterpillar haul trucks at the Marandoo mine will also be retrofitted starting mid-2018 for completion by the end of 2019.
It marks the first time AHS technology has been deployed by Rio on Caterpillar haul trucks.
"We are grateful for the opportunity to work with Rio Tinto in installing their first Cat autonomous haulage system," Caterpillar vice president with responsibility for the Surface Mining & Technology Division Jean Savage said.
"Working with WesTrac, we look forward to helping Rio Tinto enhance operations with our proven mining technology."
Rio said the projects would make a significant contribution to its US$5 billion productivity drive.
"We are excited to be starting a new chapter in our automation journey with a valued long-term partner in Caterpillar and we are proud to be extending our successful partnership with Komatsu on this world-first retrofitting initiative," Rio Tinto Iron Ore chief executive Chris Salisbury said.
"Rapid advances in technology are continuing to revolutionise the way large-scale mining is undertaken across the globe.
"The expansion of our autonomous fleet via retrofitting helps to improve safety, unlocks significant productivity gains, and continues to cement Rio Tinto as an industry leader in automation and innovation."
Salisbury said Rio was studying further additions to the autonomous fleet, based on value, to help it achieve its productivity goals.
"Rio Tinto is committed to working closely with our workforce as we transition to AHS including providing opportunities for new roles, redeployment, retraining and upskilling," he said.
It is estimated the retrofit program will help to deliver an additional $500 million of free cashflow per year from 2021.
Last year, on average, Rio's autonomous haul trucks each operated an additional 1000 hours and at 15% lower load and haul unit cost than conventional haul trucks.
Around 20% of the existing fleet of almost 400 haul trucks in the Pilbara is AHS-enabled, which will grow to 30% at the completion of the program.
Rio is automating its rail system, with the full implementation of AutoHaul due for completion by the end of next year.
