Inclusion Criteria:
* Male or female participants. Woman of childbearing potential must be ready and willing to use highly effective methods of birth control per ICH M3 (R2) that result in a low failure rate of less than 1 percent per year when used consistently and correctly.
* Age ≥18 years at screening
* Have a diagnosis of chronic plaque psoriasis (with or without psoriatic arthritis) at least 6 months before the first administration of study drug. Duration of diagnosis may be reported by the participant.
* Have stable moderate to severe chronic plaque psoriasis with or without psoriatic arthritis at both Screening and Baseline (Randomization); Have an involved body surface area (BSA) ≥ 10% and Have a Psoriasis Area and Severity Index (PASI) ≥ 12 and Have a static Physician Global Assessment (sPGA) score of ≥ 3.
* Must be a candidate for systemic therapy or phototherapy for psoriasis treatment, as assessed by the investigator
* Signed and dated written informed consent prior to admission to the study and performance of any study procedures in accordance with Good Clinical Practice (GCP) and local legislation
Exclusion Criteria:
* Participants with nonplaque forms of psoriasis (including guttate, erythrodermic, or pustular); current drug-induced psoriasis (including a new onset of psoriasis or an exacerbation of psoriasis from beta blockers, calcium channel blockers, or lithium); active ongoing inflammatory diseases other than psoriasis and psoriatic arthritis that might confound trial evaluations according to the investigators judgment.
* Previous exposure to ABBV-066
* Currently enrolled in another investigational study or less than 30 days (from screening) since completing another investigational study
* Use of any restricted medication as noted or any drug considered likely to interfere with the safe conduct of the study.
* Major surgery performed within 12 weeks prior to randomization or planned within 12 months after screening (e.g., hip replacement, removal aneurysm, stomach ligation).
* Known chronic or relevant acute infections such as active tuberculosis, human immunodeficiency virus (HIV), or viral hepatitis
* Any documented active or suspected malignancy or history of malignancy within 5 years prior to screening, except appropriately treated basal cell carcinoma or squamous cell carcinoma of the skin or in situ carcinoma of uterine cervix
* Evidence of a current or previous disease (including chronic alcohol or drug abuse), medical condition other than psoriasis, surgical procedure (i.e., organ transplant), medical examination finding (including vital signs and electrocardiogram \[ECG\]), or laboratory value at the screening visit outside the reference range that in the opinion of the Investigator, is clinically significant and would make the study participant unable to adhere to the protocol or to complete the trial, compromise the safety of the patient, or compromise the quality of the data.
* History of allergy/hypersensitivity to a systemically administered biologic agent or its excipients
* Women who are pregnant, nursing, or who plan to become pregnant while in the trial
* Previous enrolment in this trial
