Introduction: The current study aimed to develop and assess UV-spectrophotometric (zero order, first order, second order, area under the curve) and RP-HPLC methods for estimating theophylline in its pharmaceutical dosage form. Methods: A less toxic solvent composition of acetonitrile and 0.1% orthophosphoric acid (25:75 v/v) is used as a mobile phase and diluent for developing both UV-spectroscopic and RP-HPLC techniques. Shimadzu Prominence LC-20A Modular HPLC system with a C18 column (250 × 4.6 mm; 5 µm) was used to develop the RP-HPLC method. Results: Method A is a zero-order spectrophotometric method for determining theophylline at 271 nm, and the correlation coefficient in the linearity study was found to be 0.9958, LOD, and LOQ are 0.52 and 1.71 µg/mL. Method B is a first-order spectrophotometric method for determining theophylline at 258 nm, and the correlation coefficient in the linearity study was found to be 0.9983, LOD, and LOQ are 0.46 and 1.51 µg/mL. Method C is a second-order spectrophotometric method for determining theophylline at 218 nm, and the correlation coefficient in the linearity study was found to be 0.9941, LOD, and LOQ are 0.38 and 1.25 µg/mL. Method D is an area under the curve spectrophotometric method for determining theophylline at 260 to 282 nm, and the correlation coefficient in the linearity study was found to be 0.9978, LOD, and LOQ are 0.57 and 1.88 µg/mL. Method E is the RP-HPLC method for the determination of theophylline at the retention time of 2.814 min, and the correlation coefficient in the linearity study was found to be 0.9923, LOD, and LOQ are 0.78 and 2.57 µg/mL. Studies on stress degradation show that oxidation and acid degradation mostly impact theophylline solutions. Conclusion: Theophylline can be determined using the proposed approach, which is convenient, precise, affordable, and reproducible.