Lepidium sativum (LS) is an annual plant that has been used for the treatment of many ailments. The ethanol extracts of pinnately-lobed (PL), pinnately-compound (PC), and pinnately-veined (PV) leaves of LS were examined for their total polyphenol and flavonoid contents, and their phenolic acid (namely caffeic acid (CA), p-coumaric acid (PA), and ferulic acid (FA)) contents were determined using reverse-phase high-performance liquid chromatography-photodiode array (HPLC/PDA) analysis. Among the three leaf types, the highest average total polyphenol content was found in PV (79.87 mg GAE/g extract), whereas PC showed the highest average total flavonoid content (53.35 mg QE/g extract). According to the HPLC/PDA results, PV exhibited a high amount of CA (78.60 µg/g extract) and FA (1,722.85 µg/g extract), whereas a high content of PA (258.72 µg/g extract) was detected in PC. Higher amounts of the phytochemical compounds PV and PC might be indicative of their superior biological activities compared to PL. To the best of our knowledge, this study is the first to quantify and compare the total polyphenol, flavonoid, and phenolic acid contents in three different leaf types of LS.