Updated project metadata. General transcription factor TFIID is a cornerstone of RNA polymerase II transcription initiation in eukaryotic nuclei. Human TFIID is a megadalton-sized multiprotein complex composed of the TATA-binding protein (TBP) and 13 TBP-associated factors (TAFs). The cellular mechanism of TFIID assembly is poorly understood. In the cytoplasm of human cells, we discovered a heterotrimeric TFIID sub-complex consisting of the TAF2, TAF8 and TAF10 proteins. Native mass-spectrometry uncovered the interactions between the TAFs, defining a central role of TAF8 in nucleating the complex. X-ray crystallography revealed a non-canonical arrangement of the TAF8-TAF10 histone fold domains (HFDs). TAF2 binds to multiple motifs within the TAF8 C-terminal region, and these interactions dictate TAF2 incorporation into a core-TFIID complex that exists in the nucleus. Our results provide evidence for a step-wise assembly pathway of nuclear holo-TFIID, regulated by nuclear import of preformed cytoplasmic submodules.