Dandy–Walker malformation (DWM), also known as Dandy–Walker syndrome (DWS), is a rare congenital brain malformation in which the part joining the two hemispheres of the cerebellum (the cerebellar vermis) does not fully form, and the fourth ventricle and space behind the cerebellum (the posterior fossa) are enlarged with cerebrospinal fluid. Most of those affected develop hydrocephalus within the first year of life, which can present as increasing head size, vomiting, excessive sleepiness, irritability, downward deviation of the eyes and seizures. Other, less common symptoms are generally associated with comorbid genetic conditions and can include congenital heart defects, eye abnormalities, intellectual disability, congenital tumours, other brain defects such as agenesis of the corpus callosum, skeletal abnormalities, an occipital encephalocele or underdeveloped genitalia or kidneys. It is sometimes discovered in adolescents or adults due to mental health problems.

DWM is usually caused by a ciliopathic or chromosomal genetic condition, though the causative condition is only identified in around half of those diagnosed before birth and a third of those diagnosed after birth. The mechanism involves impaired cell migration and division affecting the long period of development of the cerebellar vermis. The mechanism by which hydrocephalus occurs in DWM is not yet fully understood. The condition is diagnosed by MRI or, less commonly, prenatal ultrasound. There are other malformations that can strongly resemble DWM, and disagreement exists around the criteria and classifications used for the malformation.

Treatment for most involves the implantation of a cerebral shunt in infancy. This is usually inserted in the posterior fossa, but a shunt in the lateral ventricles may be used instead or in conjunction. Endoscopic third ventriculostomy (ETV) is a less invasive option for patients older than 1 year. Posterior fossa shunts are most effective (80% of the time) but carry the highest risk of complications, while ETV is least effective but has the least risk of complications. The mortality rate is roughly 15%, mostly due to complications from hydrocephalus or its treatment, which can include subdural haematomas or infection. The prognosis after successful hydrocephalus treatment is usually good but depends on any associated condition and its symptoms. Those without hydrocephalus are treated based on any associated symptoms or condition.

The prevalence of DWM is estimated at between 1 in 25,000 to 1 in 50,000. DWM is the cause of around 4.3% of cases of congenital hydrocephalus and 2.5% of all cases of hydrocephalus. At least 21% of those with DWM have a sibling with the malformation, and at least 16% have a parent with the malformation. The malformation was first described by English surgeon John Bland-Sutton in 1887, though it was named by German psychiatrist Clemens Ernst Benda [de] in 1954 after American neurosurgeons Walter Dandy and Arthur Earl Walker, who described it in 1914 and 1942, respectively.

The most frequent and prominent symptoms of DWM are those associated with hydrocephalus in the postnatal period. Hydrocephalus occurs in an estimated 80% of patients with classic DWM. This usually presents within the first year of life (85% of the time), most often within the first three months. Signs of hydrocephalus in infants include increasing head size, vomiting, excessive sleepiness, irritability, downward deviation of the eyes (known as "sunsetting eyes") and seizures. In contrast to classic DWM, only around 30% of those with Dandy–Walker variant (DWV), in which the posterior fossa is not enlarged, have hydrocephalus.

Despite the hypoplastic cerebellar vermis, just over half of individuals with DWM (between 27% and 84%) do not appear to have significant intellectual disability or developmental delay. However, many of the genetic conditions associated with DWM can present with developmental delay and other brain anomalies. Agenesis of the corpus callosum has been found in between 5% and 17% of those with DWM. This does not seem to result in intellectual disability on its own, however. Other brain abnormalities known to be sometimes associated with DWM include grey matter heterotopia, pachygyria (fewer ridges in the brain), lissencephaly (shallower ridges), polymicrogyria, holoprosencephaly and schizencephaly. Individuals with these features tend to have developmental delay or seizures. Those without any other central nervous system abnormalities tend to have normal or close-to-normal intellectual development. A 2003 review found that moderate-to-severe intellectual disability and non-DWM brain abnormalities were only present in those with the most severe cerebellar vermis malformations (less than two fissures/three lobules in the vermis), and these comprised 16% of their sample. Hydrocephalus also affected all of these patients.

In Dandy–Walker variant (DWV) and mega cisterna magna specifically, which are less severe malformations, there appears to be an increased rate of psychotic spectrum disorders such as schizophrenia, bipolar disorder, mania or catatonia.

A 2017 review found the following associations in patients with DWS (usually from an associated genetic condition or abnormality):