The nuclear steroid hormone receptor Progesterone Receptor (PGR) is expressed in granulosa cells in the ovarian follicle in a tightly regulated pattern in response to the LH surge. PGR plays a critical role in ovulation however the mechanism for this is still poorly understood. The genome-wide binding of PGR to chromatin involves ovary-specific motif recognition and is suggested to involve unique transcription factor interactions. To characterise the transcriptional complexes formed in ovarian granulosa cells, we performed immunoprecipitation mass spectrometry in the human KGN granulosa cell line expressing PGR isoforms PGR-A or PGR-B. TRIM28, a member of the Transcriptional Intermediary Factor 1 (TIF1) family of proteins, was identified as a specific PR-interacting factor. TRIM28 is primarily thought to be a transcriptional co-repressor, however it has recently been shown to positively mediate PGR action in the uterus. Trim28 is highly expressed in the mouse ovary, is not regulated by LH but is required for the maintenance of granulosa cell identity. Publicly available ChIP-seq data from mouse ovary shows approximately 47% overlap between PGR and TRIM28 peaks. Overexpression of TRIM28 caused an upregulation of a subset of PR target genes in response to the PGR-specific agonist R5020 and conversely knockdown caused a downregulation, suggesting TRIM28 acts as a selective co-activator of PGR transcriptional regulation. This was supported using a PGR element reporter system as a measure of direct PGR-promoter activation, which confirmed TRIM28 as a PGR co-activator. This study demonstrates a role for TRIM28 in regulating PGR activity in the ovary. This mechanism provides new insights into the molecular basis for infertility as well as novel targets for development of improved contraceptives.