Plasma in or in‐contact with liquids is a research field that has great potential in the treatment of wastewater. Today, efforts are focused on finding a plasma source that can efficiently remove persistent pollutants from water. In this study, we evaluate the effectiveness of a microwave plasma jet, in TIAGO (Torche à Injection Axiale sur Guide d'Ondes) configuration, using methylene blue (MB) as a model organic contaminant. The assessments conducted herein show that MB degradation is promoted at short nozzle–water distance, high gas flow rate, elevated microwave power, and excessive initial water conductivity (up to 10,000 µS/cm). The plasma characteristics, namely composition, electron density, and OH rotational temperature, as well as water acidity and conductivity are also evaluated.