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To date, there has been limited knowledge on the applicability of the spline interpolation method to estimate coastal water quality parameters. This study aims to assess the performance of the regularised and tension spline interpolation techniques for predicting and mapping the phosphate concentration in the surface water of Pulau Tuba, Langkawi, Kedah. Sampling points were set up randomly along the coastal water of Pulau Tuba in November 2018. Samples were obtained using the Niskin Water Sampler at 1 meter below the surface and immediately transported into the marine technology laboratory. Phosphate levels were determined using an Ultraviolet Visible spectrophotometer and the ascorbic acid technique. The Geographic Positioning System (GPS) was used to record the geolocation of each sampling point. The training set (50%) and the testing set (50%) were chosen randomly based on 20 sampling points. The regularized and tension spline interpolation methods were used for this study. Evaluation of prediction of models was carried out using the Root Mean Square Error (RMSE) statistics. The study found that the tension spline interpolation method better predicted phosphate concentration. The Root Mean Square Error (RMSE) was reported at 0.106 and 0.094 for the regularized and tension spline interpolation methods, respectively. Concerned parties can utilize the study's findings as guidelines for monitoring, managing, and developing strategic strategies for the long-term development of the coastal waters of Pulau Tuba, Langkawi, Kedah.