The transcription factor, interferon regulatory factor 5 (IRF5), is important in the induction of type I interferon, proinflammatory cytokines and chemokines, and is involved in autoimmune diseases and tumourigenesis. However, the mechanisms underlying the transcriptional regulation of wild‑type IRF5 remain to be fully elucidated. The present study was primarily designed to clarify whether specificity protein 1 (Sp1) was involved in the regulation of IRF5. Initially, the IRF5 promoter region was cloned and its promoter activity was examined using Hela and HEK 293 cells. Deletion analyses revealed that the region spanning ‑179 to +62 was the minimal promoter of IRF5. Bioinformatics analyses showed that this region contained three putative Sp1 binding sites, and mutational analyses revealed that all the Sp1 sites contributed to transcriptional activity. Secondly, the overexpression of Sp1 was found to increase the activity of the IRF5 promoter and the mRNA level of IRF5, determined using reporter gene assays and polymerase chain reaction analysis, respectively. By contrast, treatment with mithramycin and Sp1 small interfering RNA significantly reduced the activity of the IRF5 promoter and the mRNA level of IRF5. Finally, the results of an electrophoretic mobility shift assay and a chromatin immunoprecipitation assay demonstrated that Sp1 bound to the promoter region of IRF5 in vitro and in vivo. These results suggested that the Sp1 transcription factor is the primary determinant for activating the basal transcription of the IRF5.