Significance Cockayne syndrome group B (CSB) belongs to the SWI2/SNF2 family chromatin remodelers and plays critical roles in DNA damage response, repair, and gene expression. Despite its broad roles in gene expression, it has remained elusive whether and how CSB functions as a chromatin remodeler, a transcription elongation factor, or both, in regulating RNA polymerase II (Pol II) transcription on chromatin. Here, we reveal that Rad26, the yeast CSB ortholog, is recruited to the upstream of Pol II as an ATP-dependent processivity factor that assists Pol II in overcoming downstream nucleosome barriers during transcription. This paper provides mechanistic insights into the roles of CSB in different aspects of transcription and DNA repair on chromatin, including release of transcription pausing and elongation.