Vaccines are crucial to ending the COVID-19 pandemic. An mRNA-based COVID-19 vaccine can cause mild to moderate side effects. A number of cases of cardiac, gastrointestinal, and psychiatric side effects have been reported as rare side effects associated with the COVID-19 vaccine. This article presents a patient, who after the second injection of the mRNA-based COVID-19 vaccine, immediately developed anxiety, nonspecific fear, and insomnia as the prodromal phase of psychosis. Starting from the second week, the patient manifested delusions of persecution, delusions of influence, thoughts insertion, and delusional behaviour, culminating in the suicide attempt. The duration of psychosis was eight weeks, and symptom reduction was observed only after the gradual administration of antipsychotics over four weeks. The investigations of the patient did not support any structural changes of the brain, any severe medical conditions, a neurological abnormality, a confusion or a state of unconsciousness or alterations in laboratory tests. Psychosis due to the use of alcohol or psychoactive substances was excluded. The psychological assessment of the patient demonstrated the endogenous type of thinking, and the patient had schizoid and paranoid personality traits strongly associated with schizophrenia. This case indicates a strong causal relationship between the mRNA-based COVID-19 vaccine injection and the onset of psychosis. We intend to follow up this case for possible development of schizophrenia and understand that the COVID-19 vaccine could possible play a trigger role in the development of primary psychosis. Longer-term supporting evidence is needed to estimate the prevalence of psychosis following vaccination with the mRNA-based COVID-19 vaccine.