PSIP1 (PC4 and SFRS1 interacting protein 1) encodes two splice variants: lens epithelium-derived growth factor or p75 (LEDGF/p75) and p52. PSIP1 gene products were shown to be involved in transcriptional regulation, affecting a plethora of cellular processes, including cell proliferation, cell survival, and stress response. Furthermore, LEDGF/p75 has implications for various diseases and infections, including autoimmunity, leukemia, embryo development, psoriasis, and human immunodeficiency virus integration. Here, we reported the first characterization of the PSIP1 promoter. Using 5' RNA ligase-mediated rapid amplification of cDNA ends, we identified novel transcription start sites in different cell types. Using a luciferase reporter system, we identified regulatory elements controlling the expression of LEDGF/p75 and p52. These include (i) minimal promoters (-112/+59 and +609/+781) that drive the basal expression of LEDGF/p75 and of the shorter splice variant p52, respectively; (ii) a sequence (+319/+397) that may control the ratio of LEDGF/p75 expression to p52 expression; and (iii) a strong enhancer (-320/-207) implicated in the modulation of LEDGF/p75 transcriptional activity. Computational, biochemical, and genetic approaches enabled us to identify the transcription factor Sp1 as a key modulator of the PSIP1 promoter, controlling LEDGF/p75 transcription through two binding sites at -72/-64 and -46/-36. Overall, our results provide initial data concerning LEDGF/p75 promoter regulation, giving new insights to further understand its biological function and opening the door for new therapeutic strategies in which LEDGF/p75 is involved.