Synthetic colourants used in pharmaceutical formulations have been associated with cancer aetiology and toxicity to some vital human organs. Therefore, this study was designed to determine the stability of natural colourant from Citrullus Lanatus in paracetamol syrup. The fleshy fruit was sliced into smaller pieces, dried, powdered and the physicochemical properties were determined using standard protocols. The powdered sample was extracted successively by maceration using ethyl acetate and ethanol, and then dried in an oven at 80o C. The solubility of the extract was tested in ethanol and water, pH and insoluble matter determinations. Paracetamol syrup was formulated and then coloured with the C. lanatus extract in comparison to amaranth (reference). The effect of light and temperature at 37o C and 52 o C on the colourants were determined over a period of 14 days. The effect of the colourants on the drug was determined by placing the syrup in desiccator at 75 % RH and 40o C. The absorbance of the samples was measured using the UV-VIS spectrophotometer. The result of the light stability study showed that C. lanatus colourant was more stable when stored in amber bottles than in plain bottles. The temperature stability studies demonstrated a gradual increase in concentration at 37o C which decreases when the temperature was raised to 52o C. The formulations used for drug stability studies showed a decrease in concentration over the study period. The study indicated that Citrullus lanatus colourant is suitable for paracetamol syrup when protected from light and high temperature