The presence of nonprominent hallucinations in delusional disorder (DD) has been accepted by the current Diagnostic and Statistical Manual of Mental Disorders, 5th ed. A recent meta-analysis revealed that patients with schizophrenia treated with long-acting atypical antipsychotics showed a significant improvement in psychotic symptoms. However, little research has been conducted on DD. Our goal was to investigate demographic and clinical differences between two subgroups of DD patients, those with nonprominent hallucinations and those without hallucinations, and to determine treatment effectiveness of long-acting antipsychotics in these patients. We conducted a longitudinal observational study with a 6-month follow-up period in a clinical group of 45 DD outpatients. Positive and Negative Syndrome Scale (PANSS), Personal and Social Performance Scale, and Hamilton Rating Scale for Depression-17 (HRSD-17) were used for assessment. Age at onset of DD, scores in baseline assessment scales, and drug compliance were included in the analysis as potential confounders. When uncorrected for influencing factors, patients treated with long-acting antipsychotics showed lower scores in PANSS positive and negative subscales. There were no statistically significant clinical subgroup×treatment group interactions for any of the scores in assessment scales at 6 months. After adjustment, patients treated with long-acting antipsychotics showed lower scores in the PANSS negative subscale and a tendency toward improvement in scores in the PANSS positive subscale. Our study suggests that risperidone long-acting injection and paliperidone palmitate long-acting injection may be useful in the treatment of DD patients, specifically those with nonprominent hallucinations.