• Ages 18 years old up to and including 65 years of age - Negative UDS results for illicit drugs at screening and prior to each drug administration session - Consent to all study procedures - Have an existing diagnosis of Prolonged Grief Disorder. May also be determined to have Complicated Grief Disorder without official diagnosis as determined by PI or designate based on DSM V criteria - Score of greater than 25 on the Inventory of Complicated Grief - Agree to abstain from any psychoactive drugs on the day prior to and the day of the drug administration session - People of childbearing potential that are sexually active must agree to continue or initiate practice of a highly effective means of birth control, alone or in combination with another, throughout the study. Highly effective options are: implants, intrauterine devices (IUD), and sterilization. Exclusive use of condoms is not effective. - Be judged by study team clinicians to be at low risk for suicidality as determined by MINI, Columbia Suicide Severity Scale, and the Patient Health Questionnaire-9. - Concurrent psychotherapy or pharmacotherapy with SSRIs, SNRIs, and/or bupropion (< 300 mg bupropion) is allowed if the type and frequency of the therapy has been stable for at least two months prior to screening and is expected to remain stable during participation in the study - Be otherwise medically stable as determined by screening for medical problems via a personal interview, a medical questionnaire, a physical examination, an electrocardiogram (ECG), urine beta-HCG, and urine toxicology screen - Participant must agree to consume approximately the same amount of caffeine-containing beverage (e.g., coffee, tea) that they consume on a usual morning, before arriving at the research unit on the mornings of drug session days. If the participant does not routinely consume caffeinated beverages, they must agree not to do so on session days - Agree not to take any PRN medications on the mornings of drug sessions - Agree not to take sildenafil (Viagra®), tadalafil, or similar medications within 72 hours of each drug administration - Agree that for one week before each drug session, participant will refrain from taking any nonprescription medication, nutritional supplement, or herbal supplement except when approved by the study investigators. Exceptions will be evaluated by the study investigators and will include acetaminophen, non-steroidal anti-inflammatory drugs, and common doses of vitamins and minerals - Willingness and ability to remain within the observation room for the duration of the study visits up to 10 hours - Willingness and ability to follow study protocol as directed by research staff - Willing and able to attend all sessions in the study and complete follow up assessments - Fluent in English - Ability to provide one photo of a deceased loved one and one photo of a living loved one, as required for the grief elicitation task.

General medical exclusion criteria: - Previous use of a psychedelic drug may be exclusionary depending on frequency, context, and participant safety as determined by the PI or designate. - Person who is pregnant, nursing, or planning to get pregnant determined at screening and before drug session by urine test or self-report - People of childbearing potential that are sexually active who are not practicing an effective means of birth control - Cardiovascular conditions: coronary artery disease, stroke, angina, a clinically significant ECG abnormality (e.g., atrial fibrillation), prolonged QTc interval (i.e., QTc > 450 msec), heart valve, or TIA in the past year. - History of head trauma with neurological deficit; seizures, or neurologic disorders including cerebrovascular disease, epilepsy, or neurogenerative diseases - Type 1 diabetes - BMI <18 - Currently taking on a regular (e.g., daily) basis any antidepressant medications other than SSRIs, SNRIs, or bupropion, or any other medications that have a primary centrally-acting serotonergic effect, including MAOIs. Bupropion dosage must be < 300 mg in order to be included - Nicotine dependence that would be incompatible with remaining in study area for the entirety of the visit - Prescribed or illicit use of benzodiazepines or opioids within 4 weeks prior to screening - Baseline blood pressure greater than 139/79 (after repeat measures) unless stable with medication as determined by PI or PI designate - Taking any muscle relaxers, antihistamines, or other medications known to cause lethargy or impair cognitive ability within one day prior to psilocybin session - Serious medical comorbidity requiring medical intervention or close supervision - History of claustrophobia - Any court mandated or legal restrictions that would impair the participant from attending all visits - Inability to follow and comply with all study procedures - Deemed unable to meaningfully or safely participate in the study - Any legal judgement toward subject determined to interfere with study attendance or jeopardize compliance with study protocol determined by PI or designate - Individuals who are unable to undergo MRI as determined by MRI pre-screening. General psychiatric exclusion criteria: - Score of less than 25 on the Inventory of Complicated Grief - Severe psychiatric disorder (other than depression) within 6 months or lifetime history of serious psychiatric or neurological disorders, including bipolar disorder, or active psychosis - Clinically significant suicidal ideation (e.g., with strong intent or means) within past 6 months or lifetime history of suicide attempt based on the MINI, Patient Health Questionnaire-9 or Columbia-Suicide Severity Scale. At any point during the study, a participant may be withdrawn from the study for concerns of suicidality and provided follow-up care by our team or a referral to care if needed. - Current or past history of meeting DSM-5 criteria for schizophrenia spectrum or other psychotic disorders, or bipolar I disorder - Current or previous history within one year of meeting DSM-5 criteria for a moderate or severe alcohol, or other drug use disorder (excluding tobacco, caffeine, and cannabis) - Nicotine dependence that would be incompatible with an individual to be nicotine free for 8-10 hours on a psilocybin session day - Have a first degree relative with schizophrenia or other psychotic disorders (except substance/medication-induced or due to another medical condition), or bipolar I disorder