This study evaluated the impact of different doses of zeolite on the emergence, yield, and chemical composition of lettuce seedlings (Lactuca sativa var. capitata). To achieve this, a pot experiment was conducted using randomised design with four replications. Zeolite was mixed with the soil substrate at rates of 1%, 5%, and 10% per 1 kg of total soil mass (5 kg), corresponding to 10, 50, and 100 g·kg−1, respectively. The control treatment consisted of a light mineral soil substrate without the addition of zeolite. The study found that zeolite significantly improved lettuce seed germination and seedling emergence rate. As the proportion of zeolite in the soil increased, both the yield of edible parts (leaves) and the biomass of the roots also increased. A beneficial effect of zeolite was observed to significantly reduce cadmium and lead concentrations in leaves. Furthermore, the addition of 10 g·kg−1 of soil also led to a reduction in the nitrate content in leaves, while higher doses (50 and 100 g·kg−1 of substrate) resulted in an increased nitrate content. The application of zeolite had no effect on the levels of flavonoids, anthocyanins, or polyphenols in lettuce leaves, nor on the concentrations of primary nutrients (N, P, K, Ca, Mg, S, and Na). However, a statistically significant relationship was observed between the application of zeolite and the increase in the iron, copper and zinc content, along with the decrease in the levels of manganese, boron, and molybdenum in the edible parts of the test plant.