<p>Barium strontium titanate (Ba_1−<i>x</i>Sr_<i>x</i>TiO₃, BST) is a well-known mixed oxide exhibiting a perovskite structure with tunable dielectric and ferroelectric properties, which vary based on the composition and temperature. In this study, Barium strontium titanate (Ba_0.6Sr_0.4TiO₃), co-doped with donor dopant (K⁺) and acceptor dopant (Mg²⁺), has been prepared by a sol–gel technique using a programmable spin coater on transparent ITO-coated glass substrates. The influence of K (@5%)/Mg (@2%, 5%, 8%, 10%) co-doping has thereafter been investigated on the structural and dielectric properties of the films. The crystallite size of the samples has been noted to decrease from 33 to 24&#xa0;nm with increasing Mg²⁺ doping concentration, which is attributed to the substitution of Ti⁴⁺ ions by smaller Mg²⁺ ions. Also the tunability of the material is noted to reduce from 69 to 61% at a frequency of 1&#xa0;kHz and 10&#xa0;V bias voltage, and the same is ascribed to the reduced grain size of the material. However, the dielectric loss is observed to decrease with augmentation in Mg²⁺ concentration which in turn leads to enhancement in the figure of merit (FOM) of the material from 101 to 290. From the results obtained in the current investigation, K/Mg co-doping in BST can be perceived to be a promising way to optimize the dielectric properties of BST thin films for their potential application in microwave tunable devices.</p>