Introduction: Suboptimal adherence to antipsychotics leads to poorer outcomes and relapse. The adherence behaviour of people may be influenced by several factors including the number of antipsychotics used and their formulation. This study aimed to identify longitudinal adherence patterns to oral and long-acting injectable (LAI) antipsychotics in monotherapy or polypharmacy through group-based trajectory modelling (GBTM). Methods: This was a retrospective cohort study that linked prescription and dispensing data of adult patients with a new antipsychotic prescribed between 2015-2019 in Catalonia (Spain). GBTM was used to classify patients following a similar longitudinal pattern of adherence. The response variable was adherence, estimated through the continuous medication availability measure (CMA), in each 30-day period during 12 months of follow-up. Baseline and treatment characteristics were used to characterize the trajectories identified. Results: Among the 7,730 patients included in the study, we identified seven clinically distinct trajectory groups of adherence to antipsychotics: “non-adherent” (19%), “low adherent” (9%), “early-decline” (6%), “mid-decline” (5%), “late-decline” (5%), “high adherent” (21%), and “fully adherent” (35%). Trajectories with better adherence were more likely to receive the prescription from a psychiatrist, receive LAIs and have previous exposure to other antipsychotics. Intermittent medication use patterns and high levels of polypharmacy were characteristics of the “low” and “high adherent” groups. Conclusions: The trajectories reflect three adherence behaviours: stable over time; patients who discontinue treatment and; patients with an intermittent refill pattern. Patients on polypharmacy should have more regular adherence monitoring and LAIs should be considered, as they appear to be associated with better adherence.