Dysregulated rRNA synthesis by RNA polymerase I (Pol I) is associated with uncontrolled cell proliferation. Here, we report a box H/ACA small nucleolar RNA (snoRNA)-ended long noncoding RNA (lncRNA) that enhances pre-rRNA transcription (SLERT). SLERT requires box H/ACA snoRNAs at both ends for its biogenesis and translocation to the nucleolus. Deletion of SLERT impairs pre-rRNA transcription and rRNA production, leading to decreased tumorigenesis. Mechanistically, SLERT interacts with DEAD-box RNA helicase DDX21 via a 143-nt non-snoRNA sequence. Super-resolution images reveal that DDX21 forms ring-shaped structures surrounding multiple Pol I complexes and suppresses pre-rRNA transcription. Binding by SLERT allosterically alters individual DDX21 molecules, loosens the DDX21 ring, and evicts DDX21 suppression on Pol I transcription. Together, our results reveal an important control of ribosome biogenesis by SLERT lncRNA and its regulatory role in DDX21 ring-shaped arrangements acting on Pol I complexes. Display omitted •SLERT is a box H/ACA snoRNA-ended lncRNA that enhances pre-rRNA transcription•DDX21 forms ring-shaped structures surrounding Pol I and inhibits Pol I transcription•SLERT binds to DDX21 and modulates DDX21 rings to evict their suppression on Pol I•SLERT-DDX21 interactions regulate differential expression of rDNAs A long non-coding RNA promotes pre-ribosomal RNA transcription by loosening the ring-shaped structure surrounding multiple RNA Pol I complexes formed by RNA helicase DDX21.