Background: To scale up the services for first-episode schizophrenia in Thailand, it is essential to understand to what extent health care seeking is delayed, and how much the delay affects the treatment outcome. Objectives: To investigate the duration of untreated psychosis (DUP) and its impact on remission in first-episode schizophrenia across the country. Methods: 276 outpatients with a first-episode schizophrenia were followed for 6 months and assessed whether they fulfilled the criteria for remission at the follow-up. The proportion of those achieving remission was compared by the DUP. The impact of DUP on remission was estimated in multivariate analyses. Results: At the follow-up, 83% (71/86) of patients who had met the criteria for symptomatic remission at the baseline achieved enduring remission, whereas 63% (119/190) of patients who had not met the criteria for symptomatic remission at baseline met it at the follow-up. The shorter the DUP, the higher the proportion of those who achieved symptomatic or enduring remission at the follow-up. The impact of DUP on symptomatic remission appeared to be significant after controlling for other factors influencing remission. Conclusion: Since the DUP would influence remission of patients with schizophrenia, early detection and intervention services should be provided in Thailand.