Reactor 4 at Hunterston B power station in North Ayrshire was one of two remaining reactors at the plant taken out of service last year.
Now, the Office for Nuclear Regulation (ONR) has given operator EDF Energy the go-ahead to return Reactor 4 to service for up to four months.
The ONR said it had focused on whether cracks seen in the reactor graphite core will compromise safety and concluded "an adequate safety case has been provided to allow a further period of operation".
Donald Urquhart, ONR deputy chief inspector, said: "We have made the decision to allow Reactor 4 at Hunterston B to return to service following a long and detailed assessment of the safety case submitted by EDF Energy."
The ONR said the longer-term condition of the reactor "remains uncertain" and EDF Energy "will need to justify safe operation" beyond the four-month period.
EDF Energy said in a statement: "We are pleased to confirm that the independent nuclear safety regulator, ONR, has given permission for EDF Energy to re-start Reactor 4 at Hunterston B power station."
The energy company said tests have "demonstrated that even in the most extreme conditions our reactors operate within large safety margins", adding the control rods will "safely shut down the reactor in all circumstances".
The core will continue to be monitored to "ensure safe operations at all times".
The safety case for returning Reactor 3 at Hunterston to service has been submitted to the ONR and the regulator said it would begin assessment shortly.
Scottish Greens MSP Ross Greer raised concerns over previous safety incidents at the plant, described by EDF Energy as minor.
The politician said these combined with the cracks "paint the picture of an ageing relic being pushed beyond its already repeatedly extended lifespan".
Friends of the Earth Scotland director Richard Dixon said: "Restarting the Hunterston reactors is definitely not worth the risk.
"Most people in Scotland will not even have noticed these reactors at Hunterston have been offline for so long as dependable renewable energy has more than made up for the difference."