Pimavanserin is claimed as the first antipsychotic drug that shows selectivity for serotonin 5-HT₂ receptors (5-HT₂Rs) and lacks of affinity for dopamine D₂ receptors (D₂Rs). Cell-based functional assays suggest that pimavanserin and antipsychotics with D₂R/5-HT₂R affinity could act as inverse agonists of 5-HT_2ARs. However, there is not evidence of such pharmacological profile in native brain tissue. 5-HT_2ARs are able to engage both canonical G_αq/11- and non-canonical G_αi1-proteins. In the present study, the response to pimavanserin of the 5-HT_2AR coupling to G_αq/11- and G_αi1-proteins was measured in membranes of postmortem human prefrontal cortex by antibody-capture [³⁵S]GTPγS binding scintillation proximity assays. Pimavanserin promoted a concentration-dependant inhibition of the 5-HT_2AR coupling to G_αi1-proteins whereas the response of G_αq/11-proteins was unaltered, suggesting inverse agonism and neutral antagonism properties, respectively. The inhibition was abolished in the presence of the selective 5-HT_2AR antagonist MDL-11,939 and was absent in brain cortex of 5-HT_2AR knock-out mice when compared to respective 5-HT_2AR wild-type animals. In conclusion, the results demonstrate the existence of constitutive 5-HT_2AR activity in human brain for the signalling pathway mediated by G_αi1-proteins. Pimavanserin demonstrates 5-HT_2AR functional selectivity and exhibits inverse agonist profile towards G_αi1-proteins, which is considered the effector pathway promoting hallucinogenic responses. In contrast, pimavanserin behaves as neutral antagonist on the 5-HT_2AR coupling to the canonical G_αq/11-protein pathway. The results strengthen the relevance of inverse agonism as potential mechanism of antipsychotic activity. Moreover, the existence of functional selectivity of 5-HT_2ARs for different G_α-proteins could contribute to better design of 5-HT_2AR-related antipsychotic drugs.