It said Reid and his delegation flew into Riyadh air base on Saturday, but did not give more details.
A British Embassy spokesman said Reid was expected to meet Saudi Crown Prince and de facto ruler Abdullah bin Abdul Aziz and Prince Sultan bin Abdul Aziz.
Saudi Arabia is a major client of Britain's arms industry and is linked to London by the multibillion-dollar Al-Yamamah contract signed in 1985, the biggest arms sale in British history.
Reid's trip comes exactly three weeks after British Prime Minister Tony Blair paid a flying visit to Saudi Arabia.
It also follows visits by the Portuguese and Spanish defence ministers to the oil-rich kingdom earlier this month.