The aim of this study was to assess the feasibility of rapid sodium MRI (23Na-MRI) for the imaging of peritoneal cancer deposits in high grade serous ovarian cancer (HGSOC) and to evaluate the relationship of 23Na-MRI with tumour cellularity. 23Na-MRI was performed at 3 T on twelve HGSOC patients using a 3D-cones acquisition technique. Tumour biopsies specimens were collected after imaging and cellularity was measured from histology. Total 23Na-MRI scan time for each patient was approximately 11 min. At an isotropic resolution of 5.6 mm, signal-to-noise ratios (SNRs) of 82.2 ± 15.3 and 15.1 ± 7.1 (mean ± standard deviation) were achieved for imaging of tumour tissue sodium concentration (TSC) and intracellular weighted sodium concentration (IWS) respectively. Tumour TSC and IWS concentrations were: 56.8 ± 19.1 mM and 30.8 ± 9.2 mM respectively and skeletal muscle TSC and IWS concentrations were 33.2 ± 16.3 mM and 20.5 ± 9.9 mM respectively. There were significant sodium concentration differences between cancer and skeletal muscle, Wilcoxon signed-rank test, P <  0.001 for TSC and P =  0.01 for IWS imaging. Tumour cellularity displayed a strong negative correlation with TSC, Spearman's rho = -0.92, P <  0.001, but did not correlate with IWS. This study demonstrates that 23Na-MRI using 3D-cones can rapidly assess sodium concentration in peritoneal deposits of HGSOC and that TSC may serve as a biomarker of tumour cellularity.