The Marchioness disaster was a collision between two vessels on the River Thames in London in the early hours of 20 August 1989, which resulted in the deaths of 51 people. The pleasure boat Marchioness sank after being hit twice by the dredger Bowbelle at about 1:46 am, between Cannon Street railway bridge and Southwark Bridge.

Marchioness had been hired for the evening for a birthday party and had about 130 people on board, four of whom were crew and bar staff. Both vessels were heading downstream, against the tide, Bowbelle travelling faster than the smaller vessel. Although the exact paths taken by the ships, and the precise series of events and their locations, are unknown, the subsequent inquiry considered it likely that Bowbelle struck Marchioness from the rear, causing the latter to turn to port, where she was hit again, then pushed along, turning over and being pushed under Bowbelle's bow. It took thirty seconds for Marchioness to sink; 24 bodies were found within the ship when it was raised.

An investigation by the Marine Accident Investigation Branch (MAIB) blamed a lack of lookouts, but their report was criticised by the families of the victims, as the MAIB had not interviewed anyone on Marchioness or Bowbelle, but relied on police interviews. The government refused to hold an inquiry, despite pressure from the families. Douglas Henderson, the captain of Bowbelle, was charged with failing to have an effective lookout on the vessel, but two cases against him ended with a hung jury. A private prosecution for manslaughter against four directors of South Coast Shipping Company, the owners of Bowbelle, and corporate manslaughter against the company was dismissed because of lack of evidence.

A formal inquiry in 2000 concluded that "The basic cause of the collision is clear. It was poor lookout on both vessels. Neither vessel saw the other in time to take action to avoid the collision." Criticism was also aimed at the owners of both ships, as well as the Department of Transport and the Port of London Authority. The collision and the subsequent reports led to increased safety measures on the Thames, and four new lifeboat stations were installed on the river.

The pleasure boat Marchioness was built in 1923 by the Salter Brothers of Oxford for Joseph Mears, a businessman whose interests included running pleasure launches on the River Thames. She was 85.5 ft (26.1 m) long and 14.5 ft (4.4 m) at the beam, and measured 46.19 gross tons. She spent most of her life on the Thames, including when she was requisitioned during the Second World War by the Thames Hospital Emergency Service, when she was stationed in Dagenham. Her service included being one of the little ships that aided in the 1940 Dunkirk evacuation. Marchioness was sold to Thames Launches in 1945, when Mears's company was wound up. She was purchased by Tidal Cruises Ltd in 1978 and the upper works were rebuilt to form an upper and lower saloon. The new upper saloon obstructed the vision from the wheelhouse, and there was, the later inquiry established, a lack of easily accessible emergency exits, particularly from the lower decks. Marchioness had seven life rafts—each of which could support twenty people—and seven lifebuoys—each of which could support two people. Registered in London, she was licensed to carry 165 passengers. From the early 1970s there was an increase in the number of boats for evening or night-time parties and discos, and Marchioness was part of this market.

Marchioness carried a crew of two: her captain was Stephen Faldo; the mate was Andrew McGowan. On the night of her sinking, she also carried two members of bar staff. Faldo and McGowan had a business partnership, Top Bar Enterprises, which provided the bar staff and drink for the party. Faldo was 29 at the time of the sinking. He had begun working on the Thames at the age of 17 and had earned his full waterman's river licence in June 1984. He began work at Tidal Cruises in 1986 and became the permanent captain of Marchioness in 1987. He had forgotten to renew his riverman's licence in the run-up to the night of the collision, and was technically not entitled to skipper the vessel that night. A later inquiry considered the lack of licence was "a minor point because he was undoubtedly qualified to do so and could have renewed his licence by paying 50 pence". McGowan, age 21, became an apprentice to a waterman in June 1986. By February 1988 he had completed courses at the Port of London Authority for chartwork and seamanship, and obtained his apprentice licence in May 1988; he joined Marchioness as a crew member around the same time.

The aggregate dredger Bowbelle was launched in 1964 by the Ailsa Shipbuilding Company of Troon, Scotland. She was 262 ft (79.9 m) long and 45 ft (13.8 m) at the beam; her deadweight tonnage was 1,850 long tons (1,880 t) and she measured 1,474.72 gross tons. Bowbelle was one of six "Bow" ships owned by East Coast Aggregates Limited; they were managed by South Coast Shipping Company Limited. East Coast Aggregates Limited (ECA) was part of the larger RMC Group, a concrete products company. Bowbelle had a crew of nine: a master, two mates, three engineers, two able seamen and a cook. The ship's captain, Douglas Henderson, aged 31, undertook a Deep Sea apprenticeship until 1978 and joined ECA in November 1987, when he became second mate on Bowsprite. In May 1989 he became the master of Bowbelle.

Vessels from ECA, including Bowbelle, had previously been involved in accidents on the Thames. In July 1981 there was a collision between Bowtrader and the passenger launch Pride of Greenwich; three months later Bowtrader was involved in a collision with Hurlingham, another pleasure cruiser owned by Tidal Cruises. Bowbelle collided with Cannon Street railway bridge in May 1982 and, the following month, was nearly in a collision with a passenger boat.