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A method was developed for determining selenium with a self-made ion-selective electrode was developed. This electrode was made by using Ag2Se as electroactive material. Optimal working conditions and interferences were investigated. The electrode exhibits good potentiometric response for Se2− ions over the concentration range from 6 × 10−7 mol · L−1 to 1 × 10−4 mol · L−1 with a Nernstian slope of 28 ± 1 mV per decade and a detection limit of about 4.5 × 10−7 mol · L−1. It was used over six months and exhibits good selectivity and sensitivity towards Se2−. The method was applied to determine selenium in biological materials. The recovery ranges between 92% and 105.5%, and the relative standard derivation is less than 3.6% (n = 6).