Cyclic guanosine monophosphorothioate analogue 1a is currently showing potential as a drug for the treatment of inherited retinal neurodegenerations. To support ongoing preclinical and clinical work, we have developed a diastereoselective synthesis via cyclization and sulfurization of the nucleoside 5′-H-phosphonate monoester, which affords the desired RP-3′,5′-cyclic phosphorothioate in 9:1 ratio to the undesired SP-diastereomer. This route was made viable as a result of the silyl protection sequence used, which achieved >80% selectivity for 2′,5′-hydroxyls over 3′,5′-hydroxyls. Finally, the chromatography-free process allowed for a scale-up, as intermediates and the final product were isolated by crystallization to give 125 g of 1a (13.8% total yield) with over 99.9% HPLC purity. © 2021 The Authors.