A simple and more sensitive spectrophotometric method is developed for determination of thorium using thorin as a chromogenic reagent in the presence of cetylpyridinium chloride (CPC) in perchloric acid. The reaction was instantaneous and complex was found stable for 168 h. A significant bathochromic shift was noted in the presence of CPC. The determination range was enhanced from 25 to 30 μg mL−1 with molar absorptivity of 2.95 × 104 L mol−1 cm−1 at 25 ± 5 °C. Sandell’s sensitivity was calculated to be 6.8 ng cm−2 at 581 nm. Relative standard deviation was reduced from 4.25 to 2.5. The interference of Ni2+, Mn2+, Sn4+, phosphate, EDTA, sulphate and tartrate has been reduced significantly in the presence of surfactant. The validity of the proposed method was tested by determining thorium in Certified Reference Materials.