Peptidylarginine deiminase (PAD) enzymes convert histone arginine residues into citrulline to modulate chromatin organization and gene expression. Although PADs are expressed in anterior pituitary gland cells, their functional role and expression in pituitary adenomas is unknown. To begin to address these questions, we first examined normal human pituitaries and pituitary adenomas and found that PAD2, PAD4 and citrullinated histones are highest in prolactinomas and somatoprolactinomas. In the somatoprolactinoma-derived GH3 cell line, PADs citrullinate histone H3, which is attenuated by a pan-PAD inhibitor. RNA-sequencing and ChIP studies show that the expression of microRNAs let-7c-2, miR-23b and miR-29c is suppressed by histone citrullination. Our studies demonstrate that these miRNAs directly target the mRNA of the oncogenes HMGA, IGF-1 and N-MYC, which are highly implicated in human prolactinoma/somatoprolactinoma pathogenesis. Our results are the first to define a direct role for PAD catalyzed histone citrullination in miRNA expression, which may underlie the etiology of prolactinoma and somatoprolactinoma tumors through regulation of oncogene expression.