This multi-center, randomized, double-blind, parallel group, placebo-controlled, proof of concept study will investigate the efficacy and safety of RG7314 in adult participants with ASD. In Stage I of the study, participants will be randomized in 2:1 to receive daily oral doses of 1.5 milligrams (mg) RG7314 or placebo for 12 weeks. After an independent safety review, the study may proceed to Stage II. In Stage II of the study, additional participants will be randomized in 2:1 to receive daily oral doses of 4 mg RG7314 or placebo for 12 weeks. After an independent safety review, Stage III will be started wherein additional participants will be randomized in 2:1 to receive daily oral doses of 10 mg RG7314 or placebo for 12 weeks. During Stage III, safety will be reviewed by independent safety review twice and if no safety signal is observed, then additional participants will be randomized in 1:1:1 either to receive 1.5 milligrams per day (mg/day) or 10 mg/day RG7314 orally or placebo for 12 weeks in Stage IV.

Inclusion Criteria:

• Participants meet Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for ASD and Tenth Revision of the International Classification of Diseases and Related Health Problems (ICD-10) criteria for Autism diagnosis
• SRS-2 (T-score) greater than or equal to (>/=) 66
• CGI-S >/=4 (moderately ill)
• Participants have an Intelligence Quotient (IQ) >/=70 (Wechsler Abbreviated State of Intelligence)
• A body mass index (BMI) between 18 to 40 kilograms per square meter (kg/m^2) inclusive
• Language, hearing and vision compatible with the study measurements as judged by the investigator
• Lives with (or has substantial periods of contact with) a caregiver who is willing and able to attend visits when required, oversee the participant's compliance with protocol-specified procedures and study medication dosing, and report on the participant's status via completion of study assessments. Any period of absence must be covered by another caregiver. Non-cohabitating caregiver(s) must spend sufficient time with the participant so that, in the opinion of the investigator, the caregiver(s) can reliably assess participant's mental status, activities and behavior.

Exclusion Criteria:

• Alcohol and/or substance abuse/dependence during the last 12 months
• A significant risk for suicidal behavior, in the opinion of the investigator
• Systolic blood pressure greater than (>) 140 or less than (<) 90 millimeters of mercury (mm Hg), and diastolic blood pressure >90 or less than (<) 50 mm Hg
• Resting pulse rate >90 or <40 beats per minute
• Use of prohibited medications or herbal remedies within 2 weeks prior to randomization, or 5 half-lives (whichever is longer)
• Initiation of a new major change in psychological intervention within 4 weeks prior to randomization. Minor changes in ongoing treatment are not considered significant
• Participation in an investigational drug or device study within 60 days prior to randomization