T7 RNA polymerase (T7 RNAP) is a single-subunit, DNA-dependent RNA polymerase encoded by bacteriophage T7, widely used for highly specific and efficient in vitro and in vivo transcription. Unlike multi-subunit bacterial RNA polymerases, T7 RNAP recognizes a short, well-defined promoter sequence (~17 bp) and initiates transcription with exceptional promoter specificity and high processivity. It is capable of producing large amounts of RNA with minimal dependence on host transcription machinery, making it a cornerstone tool in molecular biology, synthetic biology, and biotechnology. T7 RNAP is extensively utilized in systems such as the T7 expression system (e.g., pET vectors in E. coli), in vitro transcription kits, and mRNA production workflows, including vaccine development. Structurally, it shares similarity with mitochondrial RNA polymerases and functions as a robust, fast, and orthogonal transcriptional engine. Due to its efficiency and specificity, T7 RNAP remains a fundamental tool for recombinant protein expression and RNA synthesis applications.