This study aimed to explore the relationship between knowledge about psychosis, insight, and medication adherence among patients at an early stage of a schizophrenia-spectrum disorder. Eighty patients were recruited from a specialized outpatient clinic. Knowledge was assessed with the modified Knowledge About Schizophrenia Test (KAST). Medication adherence was determined with the Chinese abridged Medication Adherence Ratings Scale. Insight was measured with the abridged Scale to Assess Unawareness of Mental Disorder (SUMD). The KAST total score was correlated with the SUMD. A trend correlation between KAST subscores of cause and SUMD was found. The nonadherence rate was 43.8%. The medication-adherent patients had lower SUMD (z = -4.338, p < 0.0001) and higher KAST subscore of cause (z = -2.767, p = 0.006). These two variables explained 38.9% of the variance in adherence behavior, with SUMD being the mediator. This study highlights the importance of patients' understanding of etiology of the illness and its relationship with insight and medication adherence.