This is a multicenter (involving more than 1 study center), randomized (study medication assigned by chance), double-blind (neither the invesitigator nor the participant knows the identity of the study medication), placebo- controlled (1 of the study medications is inactive), parallel-group comparative study (different groups of participants will receive different treatments at the same time). The total duration of the study will be 78 weeks which will be comprised of: a screening period (6 weeks); an efficacy assessment period (64 weeks [with a total of 7 treatments at Weeks 0, 4, 12, 16, 28, 40, and 52]) and a follow-up assessment period (8 weeks). The efficacy assessment period will further include: a placebo-controlled treatment period (Weeks 0-12) and an active drug treatment period (Weeks 12-64). During the placebo-controlled treatment period, participants will receive ustekinumab (45 mg or 90 mg) subcutaneously (SC-into the muscles) or placebo SC. During the active drug treatment period, participants will continue treatment with 45 mg or 90 mg SC as assigned during the placebo-controlled treatment period; however, participants in the placebo group will be divided into 2 groups and will receive ustekinumab 45 mg (Placebo A) or 90 mg (Placebo B) SC. Efficacy will be evaluated primarily by analysis of psoriasis area and severity index (PASI) score. Participant safety will also be monitored.