Background: There is an urgent need to find new xanthine oxidase (XO) inhibitors with few adverse effects and potent activity, not only for treating gout but also to fight diseases associated with XO activity such as cardiovascular diseases, cancer, diabetes, and obesity. Objective: Screening of Saudi medicinal plants for XO inhibitory activity, to quantify the polyphenol–flavonoid content and to study ultra high performance liquid chromatography-electrospray ionization-mass spectroscopy (UHPLC-ESI-MS) profile of compound with best promising XO inhibitory activity among screened extracts. Materials and Methods: Sixteen methanol extracts used traditionally for treating gout and/or rheumatism were screened for total polyphenol and flavonoid contents, XO inhibitory, and antiradical activity via 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The degree of XO inhibition was determined by measuring the increase in absorbance at 295 nm associated with uric acid formation. The dose-dependent inhibition profiles of the most active plants were further evaluated by estimating the IC50 values of their corresponding extracts. The most promising XO inhibitory extract, i.e., Dodonaea viscosa extract was subjected to secondary metabolites profiling using UHPLC-ESI-MS in negative ionization mode and LC-MS analysis. Results: Among screened plants, D. viscosa leaves, Punica granatum flowers, Ruta chalepensis leaves, and Solanum incanum fruits exhibited the highest activity with an inhibition of 94.4%, 83.4%, 76.2%, and 65.7%, respectively. Extracts of R. nervosus leaves, P. granatum flowers, and D. viscosa leaves showed the highest antiradical activity in the DPPH assay with IC50 values of 25.8, 27.4, and 71.2 μg/ml, respectively. The LC/MS spectrum of D. viscosa revealed the presence of 13 known compounds along with unknown compounds which belong to flavonoid, terpene, and fatty acid derivatives class. Conclusion: The findings obtained from this study revealed that the methanolic extract of D. viscosa leaf showed the highest xanthine oxidase inhibition activity and therefore is promising species for isolating active compound between the polyphenol and flavonoid content and the XO inhibitory and radical activity.