Inclusion Criteria:  For adult subjects: Participants legally competent to read, write, and sign and give informed consent. For adolescent subjects: Informed consent of a parent(s) or legal guardian, and assent by the subjects, as required by local laws. Males and females ages 12 years and older (inclusive) at the time of consent or assent. Scalp psoriasis with a Scalp-Investigator Global Assessment of Disease (S IGA) severity of at least Moderate ('3') at Baseline. Extent of scalp psoriasis involving ≥ 10% of the total scalp at Baseline. A Psoriasis Scalp Severity Index (PSSI) of at least 6 at Baseline. An IGA of body (i.e., non-scalp) psoriasis (B-IGA) of at least Mild ('2') at Baseline. A PASI score of at least 2 (excluding palms and soles) at Baseline. Clinical diagnosis of psoriasis vulgaris of at least 6 months duration at Screening as determined by the Investigator. Stable disease for the past 4 weeks. Total overall psoriasis involvement on scalp and non-scalp areas ≤ 25% BSA (not including palms/soles) at Baseline. Total non-scalp BSA should not exceed 20%. Female subject of childbearing potential (FOCBP) must have a negative serum pregnancy test at Screening (Visit 1) and negative urine pregnancy test at Baseline (Visit 2). In addition, sexually active FOCBP must agree to use at least one form of highly effective contraception or a barrier method of contraception throughout the study according to Contraception Requirements for the protocol. Females of non-childbearing potential must either be premenarchal, post-menopausal with spontaneous amenorrhea for at least 12 months (post-menopausal status will be confirmed with FSH testing) or have undergone surgical sterilization according to Contraception Requirements for the protocol. Prepubescent females must agree to be abstinent during the study. Subjects in good health as judged by the Investigator, based on medical history, physical examination, vital signs, serum chemistry labs, hematology values, and urinalysis.  Exclusion Criteria:  Subjects who cannot discontinue treatment with therapies for the treatment of psoriasis vulgaris prior to the Baseline visit and during the study. Planned excessive exposure to treated area(s) to either natural or artificial sunlight, tanning bed, or other LED. Previous treatment with ARQ-151 or ARQ-154. Females who are pregnant, wishing to become pregnant during the study, or are breast feeding. Subjects with any serious medical condition or laboratory abnormality that would prevent study participation or place the subject at significant risk, as determined by the Investigator. Subjects who are family members of the clinical study site, clinical study staff, or sponsor, or family members residing in the same household of enrolled subjects. Any condition that in the Investigator's assessment would preclude the subject from participating in the study.