Reactor 2 stopped automatically during routine testing when an issue with an electrical system was detected.
EDF energy which operates the plant said there were no health or environmental impacts.
Paul Winkle, Torness Power Station director, said: “I believe it is important to keep you updated on any developments at Torness power station.”
He added: “I wanted to make you aware that we shut down one of our reactors earlier on Monday. This is the first unplanned shut down this year.”
“Reactor 2 automatically shut down during routine testing when an issue was detected in an electrical system. Protection equipment, which is designed to ‘fail safe’, operated to automatically shut down the reactor.
“The reactor shut down safely and cooling to the reactor was maintained at all times. There were no health or environmental impacts.”
The two reactors power enough electricity to power more than two million homes.