Psoriatic arthritis (PsA) is a type of arthritis that happens when the body's immune system attacks healthy cells and tissues causing joint pain, stiffness, and swelling. Symptoms can get worse and go away for periods of time. PsA that begins before a patient's 16th birthday is called juvenile PsA (jPsA).This study will evaluate how safe risankizumab is for the treatment of psoriatic arthritis and to assess change in disease symptoms. Risankizumab is being studied for the treatment of jPsA and adalimumab is approved for the treatment of jPsA. Participants are placed in 1 of 2 groups, called treatment arms. Each group receives a different treatment. There is a 1 in 4 chance that participants will be assigned to receive adalimumab. Approximately 40 juvenile participants with jPsA will be enrolled at approximately 30 sites worldwide. Participants will receive risankizumab and adalimumab as subcutaneous (SC) injections based on body weight. At the start of Period 1, participants are randomized to receive risankizumab or adalimumab for 24 weeks. Participants who respond to the study treatment received in Period 1, will continue to receive the same treatment in Period 2 for another 100 weeks. Those with worsening jPsA symptoms in Period 2 will be withdrawn from the study. Participants who receive adalimumab are followed for safety for 70 days after the last study treatment. Participants who receive risankizumab are followed for 140 days after the last study treatment. There may be higher treatment burden for participants in this trial compared to their standard of care (due to study procedures). Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.