Inclusion Criteria:
* PsA diagnosis of at least 3 months duration prior to the date of first screening with ClASsification of Psoriatic ARthritis (CASPAR) confirmed diagnosis at Screening.
* Have active psoriasis defined by at least 1 psoriasis lesion \>= 2 cm diameter in areas other than the axilla or groin.
* Have active arthritis defined by minimum disease activity criteria:
1. \>= 3 swollen joints (based on 66 joint counts) at Screening
2. \>= 3 tender joints (based on 68 joint counts) at Screening
* On a stable dose of methotrexate (MTX) defined as:
1. Oral or parenteral treatment \>= 3 months
2. On a stable dose with an unchanged mode of application for at least 4 weeks prior to baseline
3. Stable MTX dose of \>= 10 mg/week and \<= the upper limit of the applicable approved local label
4. Can also be on stable doses of nonsteroidal anti-inflammatory drugs, sulfasalazine and/or hydroxychloroquine as long as they are also on methotrexate
Exclusion Criteria:
* Up to 30% (approximately 66 subjects) with prior exposure to a TNF inhibitor may be enrolled if the TNF inhibitor was not discontinued due to lack of efficacy or safety concerns. Subjects must be washed out for at least 5 half-lives of these drugs prior to the Baseline visit.
* Subjects on prior adalimumab may not be enrolled in the study
* Prior exposure to other non-TNF inhibitor biological disease-modifying antirheumatic drugs (DMARDs) will be permitted if the subject is washed out at least 5 half-lives of these drugs prior to the baseline visit.
* Current treatment with traditional oral/intramuscular DMARDs, including conventional synthetic DMARDs (csDMARDs; except for concomitant treatment with sulfasalazine and/or hydroxychloroquine in addition to MTX). Oral DMARDs must be washed out for at least 5 half-lives of a drug apart from MTX prior to the Baseline visit.
a. Subject could have been exposed to prior Janus kinase (JAK) or phosphodiesterase type 4 (PDE4) inhibitors so long as they have been off therapy for at least 5 half-lives.
* Stable prescribed dose of oral prednisone or prednisone equivalent \> 10 mg/day within the 30 days of the Baseline visit.
* Intra-articular or parenteral administration of corticosteroids in the preceding 4 weeks of the Baseline visit. Inhaled corticosteroids for stable medical conditions are allowed.
* Laboratory values of the following at the Screening Visit:
1. Confirmed hemoglobin \< 9 g/dL for males and \< 8.5 g/dL for females
2. Absolute neutrophil count (ANC) \< 1500 mm\^3, (or \< 1200 cells/µL for subjects of African descent who are black)
3. Aspartate aminotransferase or alanine aminotransferase \> 1.5 x the upper limit of normal (ULN) or bilirubin \>= 3 mg/dL
4. Serum creatinine \> 1.5 x the ULN
5. Platelets \< 100,000 cells/\[mm\^3\] (10\^9/L),
6. Clinically significant abnormal screening laboratory results as evaluated by the Investigator
