The existing antipsychotic therapy is based on dopamine hyperfunction and glutamate hypofunction hypotheses of schizophrenia. Adenosine receptors (ADORA) have a neuromodulatory role and can control dopaminergic and glutamatergic systems. To elucidate the effect of ADORA polymorphisms on psychopathological symptoms and adverse effects in patients with schizophrenia on long-term antipsychotic treatment, we examined 127 nonacute schizophrenia outpatients in a cross-sectional study using the Positive and Negative Symptoms Scale, Simpson-Angus Scale, Barnes Akathisia Rating Scale, and Abnormal Involuntary Movement Scale. All patients were genotyped for 18 polymorphisms in ADORA1, ADORA2A, and ADORA3. We found an association between ADORA1 rs3766566 and psychopathological symptoms (p = 0.006), in particular, with positive psychopathological symptoms (p = 0.010) and general psychopathological symptoms (p = 0.023), between ADORA2A rs2298383 and general psychopathological symptoms (p = 0.046), and between ADORA2A rs5751876 and akathisia (p = 0.015). Haplotype analysis showed an association between ADORA1 CTCAACG haplotype and overall psychopathological symptoms (p = 0.019), positive psychopathological symptoms (p = 0.021), and akathisia (p = 0.028). ADORA2A TCCTC haplotype was associated with parkinsonism (p = 0.014). ADORA3 CACTAC was associated with akathisia (p = 0.042), whereas CACTAT was associated with akathisia (p = 0.045) and tardive dyskinesia (p = 0.023). The results of this first comprehensive study on ADORA polymorphisms in patients with nonacute schizophrenia receiving long-term antipsychotic therapy suggest an important neuromodulatory role of ADORA receptors in both psychopathological symptoms and adverse effects of antipsychotics.