Manufacturers Komatsu and Caterpillar will retrofit 48 giant trucks to allow unmanned operation, taking the total number in the region to more than 130 by 2019.
It marks the first time AHS technology has been deployed by Rio on Caterpillar haul trucks.
The FTSE 100 firm said the changes would make a "significant contribution" towards a $5bn productivity programme to streamline operations.
"Rapid advances in technology are continuing to revolutionise the way large-scale mining is undertaken across the globe", says Chris Salisbury, Rio Tinto's Iron Ore chief executive.
"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".
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"We are studying future additions to our autonomous fleet in the Pilbara, based on value, to help deliver our share of $5 billion of additional free cash flow for the company by 2021".
"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".
Last year, each of Rio Tinto's autonomous haul trucks on average operated an additional 1,000 hours and at 15% lower load and haul unit cost than conventional trucks.
After completion, due by the end of 2019, more than 130 of Rio's Pilbara fleet of nearly 400 trucks would be autonomous.
Rio is automating its rail system, with the full implementation of AutoHaul due for completion by the end of next year.
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