The modern classification of pituitary neuroendocrine tumors relies mainly on immunohistochemistry for pituitary transcription factors, hormones, and other biomarkers, including low molecular weight cytokeratins. The transcription factor GATA2 is required for development of gonadotrophs and thyrotrophs but has not been used for classification of pituitary tumors. Because of genomic paralogy of GATA2 and GATA3, we postulated that GATA3 immunohistochemistry may detect GATA2 in the adenohypophysis. We examined 151 tumors originating from Ondokuz Mayis University, Turkey (n = 83) and University Health Network, Canada (n = 68). Initially, 83 tumors (26 gonadotroph, 24 somatotroph, 17 corticotroph, 12 lactotroph, 2 poorly differentiated Pit-1 lineage tumors that expressed TSH and 2 null cell tumors) from Ondokuz Mayis University were investigated with the GATA3 monoclonal antibody L50-823. Retrospective review of the files of University Health Network identified 68 tumors (43 gonadotroph, 3 somatotroph, 2 lactotroph, 1 mammosomatotroph, 9 corticotroph, 7 poorly differentiated Pit-1 lineage tumors with TSH expression, 2 plurihormonal tumors with TSH expression and 1 null cell tumor) that were examined with the same GATA3 antibody and served as a validation cohort. All somatotroph, lactotroph and mammosomatotroph tumors and the null cell tumors were negative for GATA3. Sixty-eight (98.5%) gonadotroph tumors were positive for GATA3; 64 had diffuse reactivity. Two plurihormonal tumors with TSH expression and eight (88.8%) poorly differentiated Pit-1 lineage tumors with variable TSH expression were positive for GATA3. One of 26 (3.8%) corticotroph tumors was diffusely positive for GATA3. This study shows that GATA3 immunoreactivity is characteristic of pituitary gonadotroph and TSH-producing tumors. This finding expands the pattern of transcription factors that are used to classify adenohypophysial tumors and is important in the differential diagnosis of sellar tumors, as GATA3 expression is also a feature of primary sellar paragangliomas as well as carcinomas that may metastasize to the sella.