The ongoing global pandemic has been caused by the severe acute respiratory syndrome (SARS) CoV-2 coronavirus. The replication and transcription complex (RTC) consists of SARS-CoV-2 proteins that work together in a highly sophisticated and coordinated manner to ensure the successful replication of the virus. The main RNA polymerase of the RTC is Nsp12/RdRp and contains two active sites; one of which is responsible for viral RNA synthesis whereas the other one, termed nidovirus RdRp-associated nucleotidyltransferase (NiRAN) domain, mediates RNA processing including capping. It has recently been demonstrated that for successful RNA capping to occur, another RTC protein, Nsp9, is required to bind specifically to the NiRNA site of Nsp12. Several cryo-EM structures of Nsp12 in complex with Nsp9 and other co-factors such as Nsp7 and 8 have been determined, however, the exact biochemical details of the Nsp9-Nsp12 interaction have not been established. Interestingly, Nsp9 forms a dimer under physiological conditions and the monomer/monomer interface overlaps with the Nsp12 binding surface. In this study, we have used nuclear magnetic resonance (NMR) in combination with Biolayer interferometry (BLI) to characterise the molecular details of Nsp9 binding to the NiRAN site of Nsp12 and establish the role that Nsp9 dimerisation plays in this process. These data will contribute to a growing understanding of the mechanism of RNA replication in coronaviruses which will ultimately be essential to develop new antiviral drugs for this serious infectious respiratory disease.