It is an axiom of vertebrate behavioural endocrinology that full expression of a male behavioural phenotype depends on testicular influences during development, in adulthood, or both. Sex change in fishes challenges this necessity: behavioural changes are often rapid and greatly precede gonadal changes. However, steroid hormones can have fast actions on the nervous system, so gonadal influences on behavioural sex change cannot be excluded based solely on the speed of these changes. We report that surgical gonad removal does not prevent or discernibly alter female-to-male behavioural sex change in a protogynous coral reef fish. Male behaviour assumption is instead purely dependent on attaining social dominance. This is the first example of a vertebrate fully expressing a male behavioural phenotype without current or previous exposure to a functioning testis or testicular products.