The 1973 Westminster bombing is a car bomb that explodes at about 8:50 a.m. on Thorney Street, off Horseferry Road, in Millbank, London, on December 18, 1973.

The explosion injures up to 60 people. The bomb is planted in a car, which is known to have been stolen in London the previous night. It is parked outside Horseferry House, a building occupied by the Home Office, and opposite Thames House, which is mainly occupied by the Department of Trade and Industry, when it explodes. Both buildings, and others nearby, are extensively damaged.

Two warnings are given prior to the bombing. The first is by means of an anonymous telephone call to the offices of The Evening News at 8:22 a.m. The caller says that there is a bomb in Horseferry Road and Marshall Street and that these streets should be cleared immediately. It is presumed that the caller must have meant Marsham Street and not Marshall Street. The police are informed and start their search in these streets. While the search is in progress there, the bomb explodes in Thorney Street.

The second warning, also anonymous, is received by the telephone operator in Horseferry House at 8:45 a.m., saying that there is a car bomb outside, timed to explode in half an hour. Precautionary action is being taken when the bomb explodes only about five minutes later.

The Provisional Irish Republican Army (IRA) claims responsibility for the attack, which is assumed to be in retaliation for the jailing of the Provisional IRA Belfast Brigade members who bombed the Old Bailey earlier in the year. The day before the Westminster bombing, the IRA sends two parcel bombs that target two politicians.

The Westminster bombing is one of many IRA car bombings in Northern Ireland and England during the almost 30-year conflict in Northern Ireland known as the Troubles.

In the week following the Westminster car bombing, several more IRA bombs explode in London. On December 19, the day after the Westminster bombing, one person is injured when an IRA letter bomb explodes at a London postal sorting office. Five days later, on Christmas Eve 1973, the IRA bombs two London pubs, first the North Star public house, where six people are injured, and secondly at the Swiss Cottage Tavern, in which an unspecified number of people are injured. The last bombs explode on Boxing Day, December 26, 1973, when the Stage Door public house is bombed, injuring one person, and another bomb explodes at the Sloane Square tube station where there are no injuries.