The SFO says the probe is "into suspected corruption in the conduct of business in the Republic of Guinea by the Rio Tinto group, its employees and others associated with it".
It is asking anyone with relevant information to get in touch.
Rio Tinto says it "will fully co-operate" with the investigation.
The firm employs 50,000 people in 35 countries across six continents.
Last October the mining firm agreed to sell its entire stake in the Simandou iron ore project in the west African country to Chinese firm Chinalco for between $1.1bn and $1.3bn.
The following month, Rio Tinto said it had contacted regulatory authorities in the UK and US over certain consultancy payments made in 2011 with regard to the Simandou project.
In a statement the company said:  "Rio Tinto will fully co-operate with the Serious Fraud Office and any other relevant authorities, as it has done since it self-reported in November 2016."
The UK's Serious Fraud Office (SFO) says it has opened an investigation into British-Australian mining giant Rio Tinto Group.