Component of a nucleolar small nuclear ribonucleoprotein particle (snoRNP) thought to participate in the processing and modification of pre-ribosomal RNA (pre-rRNA). Interacts specifically with the U3 small nucleolar RNA (U3 snoRNA) (PubMed:10982864, PubMed:12381732). Binds a sub-fragment of the U3 snoRNA surrounding the B/C motif (3UBC) (PubMed:10982864, PubMed:12381732). This association with the U3BC RNA is dependent on the binding of a protein called 15.5K to the box B/C motif (PubMed:10982864, PubMed:12381732). The association of the protein with the U3BC RNA was found to be also dependent on a conserved RNA structure that flanks the box B/C motif (PubMed:10982864, PubMed:12381732). The WD domains are required for nucleolar localization and U3 small nucleolar RNAs binding. Acetylation at Lys-12 and Lys-25 by KAT2B/PCAF under stress impairs pre-rRNA processing (PubMed:26867678). Deacetylation by SIRT7 enhances RRP9-binding to U3 snoRNA, which is a prerequisite for pre-rRNA processing (PubMed:26867678). Belongs to the WD repeat RRP9 family.