Pimavanserin for Rigid-compulsive Symptoms in Autism Spectrum Disorder

Inclusion Criteria: * Participant must be at least 12 years old and no greater than 40 years old. * Participants 18 years of age or older or a legally acceptable representative must be able and willing to sign an informed consent document indicating understanding of the protocol and procedures and willingness to participate in full. For a participant who is under 18 years of age, a parent or guardian must sign an informed consent document indicating understanding the protocol and procedures and willingness to participate in full. When appropriate to the participants developmental level and age, assent will be used to indicate understanding and willingness to participant. If the participant is unable to sign the assent form, the participant must provide verbal or non-verbal confirmation of willingness to continue with study procedures. Each site will abide by their respective institutional requirements regarding consent of adult informants. For adult participants at the New York site only, a parent, caregiver, or other adult informant must sign an informed consent document indicating understanding the requirements for completing their portion of the study and their willingness to participate, due to institutional human subjects requirements. * Participant must have a diagnosis of Autism Spectrum Disorder, according to DSM-5 criteria, made by a licensed study psychiatrist or psychologist and supported by the Autism Diagnostic Observation Schedule 2 (ADOS-2) completed at screening or within the past 12 months prior to screening by an appropriately trained professional. * Participant must have a Clinical Global Impression of Severity for Repetitive Behavior of 4 or greater, as rated by the study psychiatrist or psychologist at Screening and at Baseline * Participant must have a non-verbal IQ of greater than or equal to 70 as determined by the 4-subtest Wechsler Abbreviated Scale of Intelligence. * Participant must be able to speak and understand English in order to complete study measures. * Participant must live with a parent, primary caregiver, or other adult informant who can complete study measures on the basis of spending an average of at least 4 hours per day with the participant * Parent, primary caregiver, or other adult informant must speak and understand English in order to complete study measures. * Participant must be able to self-administer study medication or have parent/caregiver be able to administer study medication. * Participant must be able to swallow study medication whole with liquid. * Participant or legally acceptable representative must be willing to continue current medication(s) and behavioral intervention(s) and to not add or change medication(s) or behavioral intervention(s) over the full course of the study. Exclusion Criteria: * Participant is judged by the Investigator to be unable to perform or comply with all study specific requirements. * Participant is an employee of an investigator with direct involvement in the proposed study or other studies under the direction of a study investigator, or is a family member of an investigator. * Participant has a history of any severe or unstable psychiatric condition (e.g., schizophrenia or other psychotic disorder, bipolar disorder, major depressive disorder) that, in the opinion of the Investigator, could confound the interpretation of the study results or put the participant at undue risk. An acute episode of a mood disorder will be considered exclusionary; a participant with a history of mild to moderate mood disorder may be included in the study under the discretion of the Investigator. Other stable psychiatric conditions are permitted at the discretion of the Investigator (e.g., attention deficit hyperactivity disorder, generalized anxiety disorder, etc.). * Participant has a current or recent history of clinically significant suicidal ideation within the past 6 months, corresponding to a score of 3 or higher (active suicidal ideation with some intent to act, without specific plan) on the Columbia Suicide Severity Rating Scale (C-SSRS), or a history of suicidal behavior within the past year, as validated by the C-SSRS at screening or Day 1. * Participant has met DSM-5 criteria for a substance abuse disorder within the last 6 months prior to Screening, except for disorders related to caffeine or nicotine. * Participant has a positive test for an illicit drug or cannabis at Screening or Baseline. Participants who test positive for cannabis and who have a valid prescription may be rescreened if they agree to abstain from the cannabis for the length of their participation in the study. The repeat test must be negative for them to participate in the study. * Participant is taking a serotonin reuptake inhibitor or antipsychotic medication. * Participant has had a change to psychotropic medications within the last 2 months * Participant has received electroconvulsive therapy (ECT) in the last 6 months. * Participant has received new-onset psychotherapy or has had a change in the intensity of psychotherapy within the 2 months prior to Screening. * Participant has known allergies, hypersensitivity, or intolerance to Pimavanserin or its excipients. * Participant has received an investigational drug or used an investigational medical device within 3 months before the planned start of study or is currently enrolled in an investigational study. * Participant has a body mass index (BMI) \<17 or \>40 at Screening. * Participant has a known history of long QT syndrome or family history of sudden death. * Participant has a history of myocardial infarction, unstable angina, acute coronary syndrome, or cerebrovascular accident (CVA) within the last 4 months. Has greater than NYHA Class 2 congestive heart failure or Class 2 angina pectoris, sustained ventricular tachycardia (VT), ventricular fibrillation, torsade de pointes, or syncope due to an arrhythmia. * Participant has a history of neuroleptic malignant syndrome/serotonin s