Atrazine (/ˈætrəziːn/ A-trə-zeen) is a chlorinated herbicide of the triazine class. It is used to prevent pre-emergence broadleaf weeds in crops such as maize (corn), soybean and sugarcane and on turf, such as golf courses and residential lawns. Atrazine's primary manufacturer is Syngenta and it is one of the most widely used herbicides in the United States, Canadian, and Australian agriculture. Its use was banned in the European Union in 2004, when the EU found groundwater levels exceeding the limits set by regulators, and Syngenta could not show that this could be prevented nor that these levels were safe.

At least two significant Canadian farm well studies showed that atrazine was the most common contaminant found. As of 2001^[update], atrazine was the most commonly detected pesticide contaminating drinking water in the U.S. Studies suggest it is an endocrine disruptor, an agent that can alter the natural hormonal system. However, in 2006 the U.S. Environmental Protection Agency (EPA) had stated that under the Food Quality Protection Act "the risks associated with the pesticide residues pose a reasonable certainty of no harm", and in 2007, the EPA said that atrazine does not adversely affect amphibian sexual development and that no additional testing was warranted. The EPA's 2009 review concluded that "the agency's scientific bases for its regulation of atrazine are robust and ensure prevention of exposure levels that could lead to reproductive effects in humans". However, in their 2016 Refined Ecological Risk Assessment for Atrazine, it was stated that "it is difficult to make definitive conclusions about the impact of atrazine at a given concentration but multiple studies have reported effects to various endpoints at environmentally-relevant concentrations." EPA started a registration review in 2013.

The EPA's review has been criticized, and the safety of atrazine remains controversial. EPA has however stated that "If at any time EPA determines there are urgent human or environmental risks from atrazine exposure that require prompt attention, we will take appropriate regulatory action, regardless of the status of the registration review process."

Atrazine is an herbicide that is used to stop pre- and post-emergence broadleaf and grassy weeds in crops such as sorghum, maize, sugarcane, lupins, pine, and eucalypt plantations, and triazine-tolerant canola.

In the United States as of 2014, atrazine was the second-most widely used herbicide after glyphosate, with 76 million pounds (34 thousand metric tons) of it applied each year, nearly identical to its usage in 1974, of 76.8 million pounds. Atrazine continues to be one of the most widely used herbicides in Australian agriculture. Its effect on corn yields has been estimated from 1% to 8%, with 3–4% being the conclusion of one economics review. In another study looking at combined data from 236 university corn field trials from 1986 to 2005, atrazine treatments showed an average increase of 5.7 US bushels per acre (0.50 m³/ha) (~400 kilograms per hectare (360 lb/acre)) than alternative herbicide treatments. Effects on sorghum yields have been estimated to be as high as 20%, owing in part to the absence of alternative weed control products that can be used on sorghum.

Atrazine was invented in 1958 in the Geigy laboratories as the second of a series of 1,3,5-triazines.

Atrazine is prepared from cyanuric chloride, which is treated sequentially with ethylamine and isopropylamine. Like other triazine herbicides, atrazine functions by binding to the plastoquinone-binding protein in photosystem II, which animals lack. Plant death results from starvation and oxidative damage caused by breakdown in the electron transport process. Oxidative damage is accelerated at high light intensity.

Atrazine's effects in humans and animals primarily involve the endocrine system. Studies suggest that atrazine is an endocrine disruptor that can cause hormone imbalance.