The plant’s operators, EDF Energy, said the move was unplanned and came about after a routine maintenance inspection at the facility, near Dunbar.
The manual shut down took place on Wednesday and officials say they hope the reactor will start up again soon.
The station’s director has issued a statement that said there were no safety, health or environmental impacts.
It is the first unplanned shut down at Torness this year.
Paul Winkle, Torness Power Station director, said: “Whilst carrying out routine maintenance in the conventional turbine part of the plant there was an issue with a valve and our operations team took prompt action and manually shut down the reactor, putting safety first.
“Cooling to the reactor was maintained at all times and there were no safety, health or environmental impacts.
“The reactor will be returned to power as soon as maintenance is satisfactorily completed.”
WWF Scotland director Lang Banks said: “This might be the first unplanned shutdown this year, but given similar incidents in the past few years it’s clear that nuclear power is showing itself to be an increasingly unreliable source of energy.
“It underlines why Scotland is right to be choosing to harness more power from renewable energy sources.
“So we can finally end our reliance on unreliable and unpopular nuclear power and fossil fuels, as we approach the Holyrood elections, we’d like to see the political parties commit to making Scotland’s the EU’s first 100 per cent renewable electricity nation.”
In February it was announced that Torness nuclear power station will have its life extended by seven years, remaining operational until 2030.
The power station started operating in 1988 and now employs 550 full-time staff plus around 180 contractors.
Its two nuclear reactors generate enough electricity to power more than two million homes, according to the French firm.