The purpose of this study is to assess the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of multiple dose oral administration of SEP-380135 in participants with schizophrenia or with a major depressive episode associated with bipolar I or II disorder or major depressive disorder (MDD).

Inclusion Criteria:

• BMI from 18.0 to 35.0 kilograms per square meter (kg/m^2) (inclusive).
• Participants with a primary diagnosis of schizophrenia (cohorts 1 to 3) or bipolar I or II disorder or MDD (cohort 4) for at least 1 year (at screening), as established by clinical review, using the DSM-5 as a reference, and confirmed using the Mini international neuropsychiatric interview (MINI).
• For cohorts 1 to 3: deemed to have residual symptoms of schizophrenia at screening (i.e., be at least "mildly ill" per CGI-S criteria [CGI-S greater than or equal to (≥) 3]) and a PANSS criteria of less than or equal to (≤) 75. For cohort 4 only: deemed to be currently experiencing an MDE. Participants must be at least "moderately ill" per CGI-S criteria (CGI-S ≥ 4).
• Ability to provide written, informed consent prior to initiation of any trial-related procedures, and ability, in the opinion of the PI, to comply with all the requirements of the trial.

Exclusion Criteria:

• Attempted suicide within 12 months prior to screening
• A disorder or history of a condition, or previous gastrointestinal conditions that may interfere with drug absorption, distribution, metabolism, excretion, gastrointestinal motility, or pH, or a history of clinically significant abnormality of the hepatic (including participants with moderate [Child-Pugh Class B] and severe [Child-Pugh Class C] hepatic impairment) or renal system (a glomerular filtration rate less than (<) 60 milliliters per minute (mL/min)), or a history of malabsorption, bowel resection, bariatric surgery or gastric band/lap band surgery, or is on medications that might interfere with gastric motility.
• Aspartate aminotransferase (AST), alanine aminotransferase (ALT), or bilirubin ≥ 2 times the upper limit of the reference ranges provided by the safety laboratory at screening, or total bilirubin ≥ 2 times the upper limit of reference (except for participants with Gilbert's syndrome or similar condition).
• Has any clinically significant unstable medical condition, clinically significant chronic disease, or any psychiatric symptom or diagnosis that in the opinion of the investigator, MM, or sponsor would pose a risk to the participant or the scientific objectives of the trial.

Note: Other protocol-specified Inclusion/Exclusion criteria may apply.

Anonymized individual participant data (IPD) that underlie the results of this study will be shared with researchers to achieve aims pre-specified in a methodologically sound research proposal. Small studies with less than 25 participants are excluded from data sharing.

Data will be available after marketing approval in global markets, or beginning 1-3 years following article publication. There is no end date to the availability of the data.

Otsuka will share data on the Vivli data sharing platform: https://vivli.org/ourmember/Otsuka/