Spinach (Spinacia oleracea L.) is a widely consumed leafy green vegetable known for its rich content of vitamins, minerals, and bioactive compounds with nutraceutical potential. This study analyzed the flavonoid content in three regionally cultivated Korean spinach varieties—Seomcho, Pohangcho, and Bomulcho—in both raw and blanched samples using high-resolution mass spectrometry (UPLC-DAD-QTOF/MS). A total of 18 flavonoids were identified and characterized, including flavonol glycosides (spinacetin, patuletin, jaceidin, axillarin) and a flavone glycoside (6:7-methylenedioxyflavone). Notably, acylated compounds such as coumaric acid and ferulic acid were observed among the spinach flavonoids. The total flavonoid content ranged from 1,112.03 to 1,368.39 mg/100 g of dry weight in the raw spinach samples, with no significant changes noted after blanching. In Seomcho, the predominant flavonoids were patuletin 3-O-(2''-O-apiosyl)gentiobioside and spinacetin 3-O-(2''-O-apiosyl)gentiobioside, while Pohangcho featured spinacetin 3-O-gentiobioside and Bomulcho highlighted axillarin 4'O-glucuronide as the major compounds. These findings provide a comprehensive profile of flavonoids in Korean spinach, reveal regional differences, and suggest their potential as functional food ingredients.