<p><i>Limax maximus</i> is one of the most widely introduced terrestrial gastropods in the world. The first record of the species in Japan was published in 2009, and its distribution range has rapidly expanded. Our aims were to estimate the habitat characteristics and diet of introduced <i>L. maximus</i> in Japan. We conducted field surveys at 21 sites in Ibaraki Prefecture, where <i>L. maximus</i> is frequently reported by citizens. We found that the habitat range of <i>L. maximus</i> is broad, occurring from rural to urbanized areas; this is distinctively different from the case of native slugs and non-native slugs previously introduced to Japan. To characterize <i>L. maximus</i> diets, we performed stable isotope analyses on individuals and the species’ potential food sources. <i>Limax maximus</i> consistently showed high δ¹³C and δ¹⁵N, meaning that it occupies a similar trophic position as predatory invertebrates such as spiders and ground beetles. Analyses of the stable isotope data by using a Bayesian mixing model (MixSIAR) suggested that <i>L. maximus</i> feeds on various food sources but mainly mushrooms. Our findings imply that <i>L. maximus</i> is a generalist with regard to habitat preference and has a flexible diet; these are likely factors in its rapid population increase. Currently, the Japanese government designates <i>L. maximus</i> as an introduced species that requires comprehensive countermeasures. We recommend sharing the species’ occurrence information with the public and cautioning farmers to be aware of this slug and its detrimental effects, so that they may take measures against its potential agricultural damage.</p>