The contract is valued at more than A$100 million (US$68 million) in aggregate over a five-year period.
The mechanical and electrical maintenance services contract includes the provision of shutdowns and scaffolding at Rio Tinto's iron ore operations in the Pilbara region of Western Australia.
Rob Velletri, managing director at Monadelphous, said: "The contract allows us to build on our long-standing relationship with Rio Tinto. Our focus will be on the delivery of safe, reliable, innovative and cost-effective solutions.
"We have seen significant growth in our maintenance division over the last two years, with a more than 50% increase in revenue. This award, which is a continuation of the provision of maintenance services to Rio, ensures a strong pipeline of future work within the iron ore sector in the Pilbara.
"We look forward to continuing to partner with the communities of Karratha, Dampier, Wickham and Roebourne, providing employment, training and business opportunities in these regions."
Monadelphous has provided services to Rio Tinto's iron ore operations for more than 25 years.