This study, consisting of two independent experiments, was conducted to optimize presowing guava seed treatments and evaluate the optimized treatments in improving germination and seedling growth of guava under heat and osmotic stresses. In the first experiment, seeds of guava cultivar White Flesh Local I were soaked in water, gibberellic acid (GA3) (0.05 and 0.1%), hydrochloric acid (HCl) (5 and 10%) and sulfuric acid (H2 SO4) (5 and 10%) for 24 and 48 h (for water and GA3), and 2 and 5 min (for HCl and H2 SO4). Seed soaking (priming) with GA3 (0.1%, at 48 h) and HCl (10%, 2 min) were the most effective treatments to improve seed germination and early seed growth of guava and were used in the second experiment. In the second experiment, treated and untreated seeds were sown in plastic boxes between two layers of filter papers maintained at osmotic potentials of 0, −1.5 and −3 MPa. The germination boxes were incubated at optimal (25 °C) and higher (32 °C) temperatures. Germination was significantly suppressed at a higher temperature and with an increase in the osmotic potential. However, seed treatments with GA3 and HCl were effective to improve the germination and seedling growth of guava under both temperature and osmotic stresses. In conclusion, chemical and hormonal seed treatments may help improve the seed germination and seedling growth of guava under heat and osmotic stresses by modulation of antioxidant enzymes and leaf proline. Seed treatment with GA3 (0.1%, 48 h) was the most effective in this regard.