BackgroundGiant prolactinoma (> 4 cm in dimension) is a rare disorder. Invasive macroprolactinoma has the potential to cause base of skull erosion and extend into the nasal cavity or even the sphenoid sinus. Nasal bleeding caused by intranasal tumor extension is a rare complication associated with invasive giant prolactinoma.We report a case of giant invasive macroprolactinoma with repeated nasal bleeding as the initial symptom.Case presentationA 24-year-old man with an invasive giant prolactinoma in the nasal cavity and sellar region who presented with nasal bleeding as the initial symptom, misdiagnosed as olfactory neuroblastoma. However, markedly elevated serum prolactin levels (4700 ng/mL), and a 7.8-cm invasive sellar mass confirmed the diagnosis of invasive giant prolactinoma. He was treated with oral bromocriptine. Serum prolactin was reduced to near normal after 6 months of treatment. Follow-up magnetic resonance imaging showed that the sellar lesion had disappeared completely and the skull base lesions were reduced.ConclusionThis case is notable in demonstrating the aggressive nature of untreated invasive giant prolactinomas which can cause a diagnostic difficulty with potential serious consequences. Early detection of hormonal levels can avoid unnecessary nasal biopsy. Early identification of pituitary adenoma with nasal bleeding as the first symptom is particularly important.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12902-023-01345-y.