Soil contamination with nitrification inhibitors (NIs) has received a lot of attention over the past years due to their potential toxic effects on plant and soil systems. However, not many actions have been taken to control and prevent such NI contaminations of soils. This study aimed to investigate the effects of symbiotic fungi in reducing the uptake of the NI 1,2,4,-triazole (TZ) in lettuce (Lactuca sativa L. cv. Siyahoo) plants at the Mohaghegh Ardabili University field. For this purpose, three symbiotic fungi (Serendipita indica, Glomus etunicatum and G. mosseae) applied separately, and four doses of TZ (0, 5, 50 and 100 mg kg-1 soil) were used in a completely randomized design with four replications. The TZ concentration in plant leaves were quantified by high-performance liquid chromatography (HPLC) analysis. Exposure to TZ at 50 and 100 mg kg-1 caused a significant (p ≤ 0.01) decrease in root colonization by fungal symbionts, chlorophyll content, and leaf fresh/dry weights. Plants colonized with symbiotic fungi had lower levels of TZ compared to non-inoculated plants, which were mostly accompanied by significant reductions in leaf concentrations of the oxidative stress indicators (malondialdehyde and hydrogen peroxide). In particular, lettuce plants inoculated with S. indica had 65% and 75% lower TZ levels (at 50 and 100 mg kg-1, respectively) compared to the corresponding non-inoculated plants. The study highlights the adverse impact of TZ on plant growth and health, and underscores the potential of symbiotic fungi as effective measures to alleviate the phytotoxic effects of TZ.