An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or marine water systems. It may be a benign or harmful algal bloom.

Algal bloom is often recognized by the discoloration in the water from the algae's pigments. The term algae encompasses many types of aquatic photosynthetic organisms, both macroscopic multicellular organisms like seaweed and microscopic unicellular organisms like cyanobacteria. Algal bloom commonly refers to the rapid growth of microscopic unicellular algae, not macroscopic algae. An example of a macroscopic algal bloom is a kelp forest.

Algal blooms are the result of a nutrient, like nitrogen or phosphorus from various sources (for example fertilizer runoff or other forms of nutrient pollution), entering the aquatic system and causing excessive growth of algae. An algal bloom affects the whole ecosystem.

Consequences range from benign effects, such as feeding of higher trophic levels, to more harmful effects like blocking sunlight from reaching other organisms, causing a depletion of oxygen levels in the water, and, depending on the organism, releasing toxins into the water. Yet, algae also play a crucial role by producing about 70 % of Earth's oxygen, which supports terrestrial life. Blooms that can injure animals or the ecology, especially those blooms where toxins are secreted by the algae, are usually called "harmful algal blooms" (HAB), and can lead to fish die-offs, cities cutting off water to residents, or states having to close fisheries. The process of the oversupply of nutrients leading to algae growth and oxygen depletion is called eutrophication.

Algal and bacterial blooms have persistently contributed to mass extinctions driven by global warming in the geologic past, such as during the end-Permian extinction driven by Siberian Traps volcanism and during the biotic recovery following the mass extinction (by delaying the recovery).

The term algal bloom is defined inconsistently depending on the scientific field, and can range from a "minibloom"^{[when defined as?]} of harmless algae to a large, harmful bloom event. Since algae is a broad term including organisms of widely varying sizes, growth rates, and nutrient requirements, there is no officially recognized threshold level as to what is defined as a bloom. Because there is no scientific consensus, blooms can be described and quantified in several ways: measurements of new algal biomass, the concentration of photosynthetic pigment, quantification of the bloom's negative effect, or relative concentration of the algae compared to the rest of the microbial community. For example, definitions of blooms have included:

Blooms are the result of a nutrient needed by the particular algae being introduced to the local aquatic system. This growth-limiting nutrient is typically nitrogen or phosphorus, but can also be iron, vitamins, or amino acids. There are several mechanisms for the addition of these nutrients to the water. In the open ocean and along coastlines, upwelling from both winds and topographical ocean floor features can draw nutrients to the photic, or sunlit zone of the ocean. Along coastal regions and in freshwater systems, agricultural, city, and sewage runoff can cause algal blooms.

Algal blooms, especially large algal bloom events, can reduce the transparency of the water and can discolor it. The photosynthetic pigments in the algal cells, like chlorophyll and photoprotective pigments, determine the color of the algal bloom. Depending on the organism, its pigments, and the depth in the water column, algal blooms can be green, red, brown, golden, or purple. Bright green blooms in freshwater systems are frequently a result of cyanobacteria (colloquially known as "blue-green algae") such as Microcystis. Blooms may also consist of macroalgal (non-phytoplanktonic) species. These blooms are recognizable by large blades of algae that may wash up onto the shoreline.