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."

Morgan King, a single mother, had apologised to her professor for being absent due to childcare problems.
The University of Tennessee professor emailed to say she could bring her daughter, Korbyn, to lessons and she would be "delighted" to hold her.
Ms King said the warm words made her cry.
In an interviewed with US news channel WBIR-TV, she said: "I just didn't know what else to do. I emailed my professor after class and was apologising for missing class that day."
Professor Sally Hunter, from Knoxville, then wrote back with her "very serious" offer of help.
"Let me know if there are any other ways I could be supporting you," she said, before signing off.
Ms King shared the email on Twitter and it was shared thousands of times.
On Wednesday, the university's chancellor, Beverly Davenport, responded: "Morgan, thanks for showing us challenges college students face. Prof Hunter, thanks for being part of the solution."
Ms King said her professor, who teaches in the Department of Child and Family Studies, is not on Twitter and is nonplussed by the reaction.
"She just doesn't think she did anything special, which speaks about the kind of person she is," Ms King told WBIR-TV.
The student said she has not needed to take her up on the offer yet.
When she graduates she hopes to become a recreational therapist, working with children with disabilities.
"I have to get my degree for Korbyn, not just for me. It means a lot that people are really supportive," Ms King said.

The 96-tonne crane exploded into a fireball just before 21:30 BST on Wednesday between J2 for Beaconsfield and J1a for the M25.
A number of cars were trapped for several hours. The road partially re-opened on Thursday morning.
Highways England said one lane would remain closed for safety reasons.
Updates on this and other stories from Buckinghamshire
It said the "intense fire" happened after a "significant spillage of hydraulic fluid", causing "serious damage to the road surface", which is due to be resurfaced on Thursday night.
A specialist lifting team removed the crane from lane one and the hard shoulder earlier.
Drivers near Beaconsfield are being advised to allow extra time for their journeys.
The UK's Serious Fraud Office (SFO) says it has opened an investigation into British-Australian mining giant Rio Tinto Group.
[NEXT_CONCEPT]
A US student with a three-month-old baby has expressed gratitude to a teacher who offered to babysit so she would not miss classes.
[NEXT_CONCEPT]
Cars were trapped on the southbound M40 in Buckinghamshire overnight after a crane burst into flames, closing the motorway,