Abstract Background The randomized, phase 2 GALAXI 1 study evaluated efficacy and safety of guselkumab, a dual-acting IL-23p19 subunit inhibitor, in participants with moderately to severely active Crohn’s disease. Efficacy and safety through 5 years are reported. Methods After completing the 48-week, treat-through GALAXI 1 main study, participants could enter the long-term extension (LTE) and continue the subcutaneous maintenance regimen assigned at randomization. Efficacy for the combined guselkumab dose groups was analyzed by (1) as-observed analysis of LTE participants, (2) nonresponder imputation (NRI) analysis of LTE participants, and (3) NRI analysis of primary efficacy analysis set (all [week 0] randomized participants). The study was not designed for statistical comparisons between groups. Results Of 185 guselkumab-randomized participants in the primary efficacy analysis set, 151 participated in the LTE. Among those continuing treatment, with available data (as observed), 97.7% (n = 85 of 87) achieved clinical remission, 71.3% (n = 62 of 87) achieved endoscopic response, and 51.7% (n = 45 of 87) achieved endoscopic remission at week 240. Results from NRI analyses also showed durability of efficacy (eg, week-240 clinical remission: 59.2% [n = 84 of 142], LTE analysis set; 46.0% [n = 81 of 176], primary efficacy analysis set). High rates of clinical and endoscopic remission at week 240 were maintained in biologic-naive participants (97.8% [n = 45 of 46] and 54.3% [n = 25 of 46], respectively [as-observed]) and participants with an inadequate response or intolerance to biologic therapy (97.1% [n = 34 of 35] and 54.3% [n = 19 of 35], respectively [as-observed]). Event rates of serious adverse events, discontinuations due to adverse events, and serious infections were low in guselkumab-treated participants. Conclusions Long-term guselkumab treatment showed sustained clinical and endoscopic efficacy through week 240. The long-term safety data were consistent with those previously presented in approved indications. Clinical trial registration A Study of the Efficacy and Safety of Guselkumab in Participants With Moderately to Severely Active Crohn’s Disease (GALAXI), NCT03466411, https://clinicaltrials.gov/study/NCT03466411