The training, including weapons and public order, is organised by the British Military Mission to the Saudi Arabian National Guard, according to documents obtained under the Freedom of Information Act.
The secretive group is said to consist of 11 British Army personnel under the command of a brigadier.
The disclosure, which brought criticism from human rights groups, comes amid criticism that the government has not been as tough in tackling the Bahraini and Syrian regimes as it has been in taking on Colonel Muammar al-Gaddafi in Libya.
Nicholas Gilby, of the group Campaign Against the Arms Trade, said: "Britain's important role in training the Saudi Arabian National Guard in internal security over many years has enabled them to develop tactics to help suppress the popular uprising in Bahrain."
The Ministry of Defence stressed that British involvement in training foreign forces was intended to engender a culture of respect for human rights.
A spokesman said: "The UK provides world-class defence training and education to many countries, including in the Gulf, creating lasting ties between our armed forces and enhancing their ability to work together towards regional security and stability.
"The Gulf states are key partners in the fight against terrorism and the proliferation of nuclear weapons, as well as being an emerging source of economic and political influence.
"By providing training for countries to the same high standards used by UK armed forces we help to save lives and raise awareness of human rights."
Defence Secretary Liam Fox said: "We train a lot of other forces in things like communications and bomb disposal, including the Saudis, who are a major strategic partner, a major partner in the battle against terrorism.
"Although elements in the Saudi National Guard have been used in Bahrain, there is absolutely no evidence that they have been used for anything other than the protection of infrastructure."