Trichoderma harzianum is a species of filamentous fungi that belongs to the genus Trichoderma. It is known for its ability to promote plant growth and stimulate the plant's immune response.

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Trichoderma colonizes plant roots and are able to grow on the roots as the root system develops. They have evolved numerous mechanisms for plant enhancement and allowing plants to develop bigger root systems making the plant more adaptive to abiotic stresses such as drought. In addition, Trichoderma solubilizes nutrients present in the soil supporting the plant in nutrient uptake while receiving energy in return.

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This species of Trichoderma has been extensively studied for its beneficial role in agriculture. It can produce several plant growth-promoting compounds, such as indole acetic acid, gibberellins, and cytokinins, which can enhance root and shoot growth, increase nutrient uptake, and improve plant health.

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In addition to its agricultural applications, T. harzianum has been studied for its potential in bioremediation, as it can degrade a wide range of pollutants, including polycyclic aromatic hydrocarbons, chlorinated compounds, and pesticides. T. harzianum is a ubiquitous fungus found in soil and can be easily cultured in the laboratory. Its genome has been sequenced, and several genetic tools have been developed to study its biology and molecular mechanisms.