In an effort to reduce the accumulation of synthetic plastics in the environment, new materials are increasingly being researched and developed that, once they end up in the environment, will be completely degradable. Therefore, biodegradable mixtures of thermoplastic starch (TPS) and polylactide (PLA) were prepared in this work. TPS/PLA polymer mixtures with different compositions of individual components, without and with the addition of citric acid (LK) were prepared by mixing in a Brabender mixer. TPS was prepared by plasticizing potato starch with a proportion of 40 wt. % of glycerol in a single-screw extruder. The proportion of citric acid was 5 and 10 wt. %. The influence of citric acid on thermal, mechanical, and barrier properties was investigated, as well as an insight into the morphological structure of TPS/PLA polymer mixtures. The structure of polymers and mixtures, as well as esterification with citric acid, was confirmed by the combined technique of infrared spectroscopy with Fourier transforms and attenuated total reflection, FTIR-ATR spectroscopy. Insight into the morphological structure was investigated using scanning electron microscopy (SEM), thermal properties and degree of crystallinity of the TPS/PLA mixture using differential scanning calorimetry (DSC) and thermal stability using thermogravimetric analysis (TGA), and water absorption was tested as an indicator of barrier properties. The mechanical properties of the mixtures were determined by tensile testing. On the SEM micrographs of the TPS/PLA mixture, weak adhesion between TPS and PLA is visible, while the addition of citric acid improves the adhesion, which affects the better homogeneity, that is, the mutual miscibility of the TPS/PLA mixture. DSC analysis confirmed the action of citric acid as well as a possible compatibilizer. Also, the greater part of TPS in TPS/PLA blends affects the increase in the degree of crystallinity of PLA. With the addition of citric acid of 5 wt. % in the TPS/PLA blends, an improvement in mechanical properties and a decrease in the water absorption value are visible. Based on all the conducted tests, the TPS/PLA 50/50 polymer blend with 5 wt.% citric acid stood out as a good candidate for further research as a promising material.