Despite a well characterized mechanism, myasthenia gravis (MG) remains a dilemma in terms of etiology. Several case reports and series of cases suggest a potential cause-effect relation between SARS-CoV-2 infection or vaccination and MG. We present the case of an autoimmune MG occurring post Covid-19 in an elderly male, vaccinated with three doses of the BNT162b2/Pfizer-BioNTech vaccine. The 78-year-old male was admitted in the Neurology Clinic in early November 2021 with double vision, bilateral ptosis, dysphonia and dysphagia, 16 days after receiving a third dose of the BNT162b2/Pfizer-BioNTech SARS-CoV-2 vaccine and 12 days after testing positive for SARS-CoV-2 infection. The symptoms began to emerge at 9 days after COVID-19 diagnosis. Clinical neurological examination included ice-pack test and intramuscular neostigmine, both with positive results. Myasthenia gravis positive diagnosis was confirmed by slow repetitive nerve stimulation and abnormally increased serum levels of antibodies against acetylcholine receptors. Due to patient's refusal of further hospitalization, he was discharged with therapy recommendations. Under treatment with oral pyridostigmine, but no oral corticosteroid due to therapeutic noncompliance, the patient was readmitted two months later with aggravated symptoms. The myasthenic crisis was successfully treated with intravenous immunoglobulins, corticosteroid therapy and oral pyridostigmine. The novelty of the current case resides in the fact that, to the best of our knowledge, appears to be the first case of MG clinically manifested after COVID-19 infection in a fully vaccinated patient.