Psidium friedrichsthalianum (Myrtaceae) is a small tree with antioxidant activity in its fruits and antimicrobial activity in its leaves and thin branches. The present study analyzed the seasonal variability in the yield and essential oil composition of a P. friedrichsthalianum population in Belém, Brazil. Essential oils were obtained by hydrodistillation and analyzed by gas chromatography (GC) coupled to mass spectrometer (MS) and flame ionization detector (FID). Chemometric analyses were carried out to verify the climatic influence on the production and composition of the essential oil. The average oil yield in the dry season (August–February) was 0.5 ± 0.0%, and in the rainy season (March–May), it was 0.8 ± 0.0%, with statistical differentiation. There was a moderate correlation between oil yield and the collection area’s relative humidity (r = 0.63). The PCA and HCA analyses did not show differentiation between the P. friedrichsthalianum oil samples during the dry and rainy seasons. However, the class of monoterpene hydrocarbons presented a negative correlation with temperature (r = −0.81) and humidity (−0.80) of the sampled area. In the PCA and HCA studies, the samples were classified into three groups: Group I (leaf oils) was characterized by a higher content of α-pinene (6.3–18.0%), β-elemene (9.9–14.8%), caryophyllene oxide (4.3–16.3%), and β-pinene (4.8–13.4%). Group II (leaf oils) was defined by a higher content of selin-11-en-4-α-ol (4.6–15.6%), β-elemene (9.9–14.8%), α-pinene (6.3–18.0%), and E-caryophyllene (3.1–8.7%). Group III (fruits volatile concentrate) was characterized by a higher content of α-pinene (17.6%), α-terpineol (13.7%), and selin-11-en-4-α-ol (10.0%). There was significant seasonal variability in P. friedrichsthalianum, whose responses are directly linked to abiotic factors such as precipitation, insolation, humidity, and temperature.