Highlights • Auxin-inducible degradation discriminates direct roles of human SPT6 in transcription • Acute loss of SPT6 globally impairs RNAPII processivity and speed • SPT6 is required for efficient transcription termination on protein-coding genes • Long-term loss of SPT6 ultimately results in cryptic intragenic transcription Summary SPT6 is a histone chaperone that tightly binds RNA polymerase II (RNAPII) during transcription elongation. However, its primary role in transcription is uncertain.