In this article, we provide direct evidence that the evolutionarily conserved transcription elongation factor TFIIS functions during preinitiation complex assembly. First, we identified TFIIS in a mass spectrometric screen of RNA polymerase II (Pol II) preinitiation complexes (PICs). Second, we show that the association of TFIIS with a promoter depends on functional PIC components including Mediator and the SAGA complex. Third, we demonstrate that TFIIS is required for efficient formation of active PICs. Using truncation mutants of TFIIS, we find that the Pol II-binding domain is the minimal domain necessary to stimulate PIC assembly. However, efficient formation of active PICs requires both the Pol II-binding domain and the poorly understood N-terminal domain. Importantly, Domain III, which is required for the elongation function of TFIIS, is dispensable during PIC assembly. The results demonstrate that TFIIS is a PIC component that is required for efficient formation and/or stability of the complex.