A fatberg is a rock-like mass of waste matter in a sewer system formed by the combination of flushed non-biodegradable solids (such as wet wipes) with fat, oil, and grease (FOG) deposits. The handling of FOG waste and the buildup of its deposits are a long-standing problem in waste management, with "fatberg" a more recent neologism. Fatbergs have formed in sewers worldwide, with the rise in usage of disposable (so-called "flushable") wet wipes. Several prominent examples were discovered in the 2010s in Great Britain, their formation accelerated by aging Victorian sewers. Fatbergs are costly to remove, and they have given rise to public awareness campaigns about flushable waste.

Fatbergs form at the rough surfaces of sewers where the fluid flow becomes turbulent. In pipes and tubes with smooth inner linings, fluid near the containing wall flows only slightly slower than fluid in the central channel of the pipe; thus, the whole volume of fluid flows smoothly and freely. When fluid encounters an obstruction, a resulting swirl of water starts trapping debris. Fatbergs occur in sewer systems around the globe, in cities and smaller towns.

An obstruction can be any type of rough surface capable of snagging debris. In brick or concrete sewers there may be surplus cement drips, damaged brickwork, or loose mortar joints damaged by frost heave. In any sub-surface pipe, even of the most advanced design, penetration by foreign intrusions such as tree roots is a commonplace cause of a fatberg blockage.

Fatbergs are not just the result of fats that have congealed through cooling. The lipids in fatbergs have undergone a process of saponification. Fatbergs thus require four main components: calcium, free fatty acids, FOG, and water. Comprising not only wet wipes and fat, fatbergs may contain other items that do not break apart or dissolve when flushed down the toilet, such as sanitary napkins, cotton buds, needles, condoms, and food waste from garbage disposal units washed down kitchen sinks. The resulting lumps of congealed material can be as strong as concrete, and require specialist equipment to remove. In the United States, almost half of all sewer blockages are caused by grease, combined with the evergrowing use of wipes that end up in sewer systems.

Fatbergs can cause blockages in sewer systems. Giant fatbergs have blocked sewers in London, New York, Denver, Valencia, and Melbourne. Blocked fat reacts with the lining of the pipe and undergoes saponification, converting the oil into a solid, soap-like substance. Grease and fat blockages can cause sanitary sewer overflows, in which sewage is discharged into the environment without treatment.

Fatbergs have been considered as a source of fuel, specifically biogas. Most of the fatberg discovered in Whitechapel in London in 2017, weighing 130 tonnes (128 long tons; 143 short tons) and stretching more than 250 metres (820 ft), was destroyed, but some of it was converted into biodiesel.

In the UK, sterilized fatberg treated with a specialized bacteria produces a chemical with a pine-like smell, which can be used as an ingredient in perfumes.

Fatbergs can be mitigated through public awareness campaigns about flushable waste and grease traps for filtration at the source. Many U.S. municipalities require restaurants and food processing businesses to use grease interceptors and regulate the disposal of FOG in the sewer system. Campaigns have been launched against wet wipes because of their effect on sewer systems, including by Surfers Against Sewage, the Marine Conservation Society and other environmental NGOs who called on the UK's Advertising Standards Authority to end “misleading” branding and packaging.