The aim of this study is first to identify the major phenolic compound of a crude ethanol extract of Psidium guajava leaves collected in Vietnam and to develop validated methods for quantification of phenolic and triterpenic components by HPLC-PDA-HRMS. The major phenolic compound was determined as quercetin-3-O-sulfate which is isolated and quantified for the first time in P. guajava. Validated HPLC-DAD quantification methods were developed to quantify the major triterpenic and phenolic derivatives of this extract and found to be accurate in the concentration range of 2–50 μg/mL for phenolic, and 5–100 μg/mL for triterpenic compounds. Four crude ethanol extracts of guava leaves collected at different periods of the year were analyzed using the developed methods and were found to be richer in triterpenic than phenolic derivatives. We also observed that the weather or rainfall influenced the richness in bioactive compounds in guava leaves.