The four locations in England and Wales are owned by the Nuclear Decommissioning Authority, which looks after some of Britain's earliest nuclear sites.
Prime Minister Rishi Sunak has promised to invest in a new generation of nuclear power as Britain seeks ways of boosting its energy supplies.
Rolls-Royce said it hoped its first small modular reactors (SMRs) could come online by the early 2030s.
It plans to use public funding to build multiple reactors on each site, predicting it could create 40,000 jobs in England and Wales.
Tom Samson, the chief executive of Rolls-Royce SMR, called it an opportunity to “bring new nuclear power back to nuclear communities”.
“The sooner that work can begin at site, the sooner we can deliver stable, secure supplies of low-carbon nuclear power from SMRs designed and built in the UK,” he said.
The four sites being prioritised are Trawsfynydd, Sellafield, Wylfa and Oldbury, all the sites of defunct nuclear power stations.
A statement from Rolls-Royce said the four could generate about 15 gigawatts of electricity between them. It has a target of generating 24GW from small nuclear by 2050.
For comparison, UK wind turbines recently generated a combined 20GW for the first time, with a 50GW target by 2030.
Rolls-Royce is also investigating the site of a fifth decommissioned plant, Berkeley.
It has competitors in the field of small nuclear, including Britain's U-Battery, which is focusing on industry rather than the power grid.
David Peattie, the head of the decommissioning authority, said the Rolls-Royce study was a “tangible step forward” in freeing up land for future use.
“We’re engaging with several potential partners to explore the use of land in our estate whilst utilising the NDA’s nuclear sector expertise to support the delivery of the UK government’s energy security strategy,” he said.
Small reactors are popular with ministers because they are cheaper and take less time to build than conventional plants.
However, Britain could also build new full-sized nuclear plants such as the Sizewell C site championed by former prime minister Boris Johnson.
Downing Street this week denied reports that the project could be scrapped, instead saying that negotiations on funding were ongoing.
Another planned reactor, Hinkley Point C in the west of England, has had its construction delayed by the pandemic.
Mr Sunak this week promised to “invest in building new nuclear power stations for the first time since the 1990s” as part of Britain's revamped energy strategy.