The nuclear steroid hormone receptor Progesterone Receptor (PR) is expressed in granulosa cells in the ovarian follicle in a tightly regulated pattern in response to the LH surge. PR plays a critical role in ovulation however the mechanism for this is still poorly understood. The genome-wide binding of PR to chromatin involves ovary specific motif recognition, suggested to involve unique transcription factor interaction. To characterize the transcriptional complex formed in ovarian granulosa cells, we performed immunoprecipitation mass spectrometry in the human KGN granulosa cell line expressing PR isoforms PR-A or PR-B. A remarkable degree of specificity in PR-A vs PR-B nucleated transcriptional complexes was observed. TRIM28 and TRIM33, two members of the Transcriptional Intermediary Factor 1 (TIF1) family of proteins, were identified as specific PRA-interacting factors. TRIM28 is primarily thought to be a transcriptional co-repressor, however has recently been shown to positively mediate PR action in the uterus. Overexpression of TRIM28 and TRIM33 caused an upregulation of a subset of PR target genes in response to the PR-specific agonist R5020 and conversely knockdown of both proteins caused a downregulation, while others were unaffected, suggesting both TRIM28 and TRIM33 act as selective co-activators of PR. This was supported using a PR element reporter system as a measure of direct PR activation, which confirmed that TRIM28 is a PR co-activator. This study demonstrates a novel role for TIF1 family proteins in regulating PR activity in the ovary. This mechanism may provide new insights into the molecular basis for infertility as well as novel targets for development of improved contraceptives.