The RHOA G17V mutation is highly recurrent in follicular helper T-cell (TFH) lymphoma of the angioimmunoblastic type (AITL) (60-70% of cases) and frequently associated with mutations in other T-cell receptor signaling genes, including CD28. Here, we sought to elucidate how RHOA and CD28 variants may work in concert to sustain T-cell activation by generating stable Jurkat T-cell lines expressing wild type (wt) RHOA or RHOA G17V with wt CD28 or CD28 T195P. Concomitant expression of RHOA G17V and CD28 T195P induced significantly higher levels of interleukin 2 (IL-2) production and NFAT and AP1 transcriptional activities than either variant alone upon T-cell activation with agonistic anti-CD3 and anti-CD28 antibodies. We identified the histone acetyltransferase p300 as a major interacting partner of RHOA G17V in our model and human primary T cells. p300 inhibition abolished the increased IL-2 secretion induced by CD3/CD28 stimulation in cells expressing RHOA G17V and/or CD28 T195P. Chromatin immunoprecipitations and immunofluorescence staining revealed an increase of p300-specific H3K18ac and H3K27ac marks at the IL-2 promoter and at the whole genome level, respectively, in cells expressing RHOA G17V. Finally, immunofluorescence staining of tumor samples from four AITL patients carrying RHOA G17V variant and four AITL patients carrying wild-type RHOA showed that neoplastic TFH cells with RHOA G17V have increased H3K18ac and H3K27ac levels as compared to non-neoplastic T cells. Collectively, these findings uncover a new mechanism of action by which RHOA G17V potentiates CD28 T195P-induced NFAT and AP1 transcriptional activities by enhancing p300 histone acetyltransferase activity (HAT) and expand the notion that epigenetic deregulation contributes to the pathogenesis of TFH lymphomas.