People with a psychiatric diagnosis smoke at high rates, yet are rarely treated for tobacco use. Health care systems often use a 'no treatment' default for tobacco, such that providers must actively choose (opt-in) to treat their patients who express interest in quitting. Default bias theory suggests that opt-in systems may reinforce the status quo to not treat tobacco use in psychiatry. We aim to conduct a pilot study testing an opt-out system for implementing a 3A's (ask, advise, assist) tobacco treatment model in outpatient psychiatry. We will use a mixed-methods, cluster-randomized study design. We will implement a tobacco use clinical reminder for outpatient psychiatrists at the VA New York Harbor Healthcare System. Psychiatrists (N = 20) will be randomized 1:1 to one of two groups: (1) Opt-In Treatment Approach: Psychiatrists will receive a reminder that encourages them to offer cessation medications and referral to cessation counseling; or (2) Opt-Out Treatment Approach: Psychiatrists will receive a clinical reminder that includes a standing cessation medication order and a referral to cessation counseling that will automatically generate unless the provider cancels. Prior to implementation of the reminders, we will hold a 1-hour training on tobacco treatment for psychiatrists in both arms. We will use VA administrative data to calculate the study's primary outcomes: 1) the percent of smokers prescribed a cessation medication and 2) the percent of smokers referred to counseling. During the intervention period, we will also conduct post-visit surveys with a cluster sample of 400 patients (20 per psychiatrist) to assess psychiatrist fidelity to the 3 A's approach and patient perceptions of the opt-out system. At six months, we will survey the clustered patient sample again to evaluate the study's secondary outcomes: 1) patient use of cessation treatment in the prior 6 months and 2) self-reported 7-day abstinence at 6 months. At the end of the intervention period, we will conduct semi-structured interviews with 12-14 psychiatrists asking about their perceptions of the opt-out approach. This study will produce important data on the potential of opt-out systems to overcome barriers in implementing tobacco use treatment in outpatient psychiatry. Clinicaltrials.gov Identifier NCT04071795 (registered August 28, 2019). https://www.clinicaltrials.gov/ct2/show/NCT04071795.