Slowly degrading microplastic (MP) is a gradually increasing pollutant in various ecosystems. To date, a wealth of information is available for aquatic ecosystems, while the attention to MP in terrestrial ecosystems is just increasing. Ubiquitous MP may enter food webs via unintentional ingestion by the soil macrofauna, but consequences are not well known. The effects on the macrofauna could be direct or indirect through changes of the gut microbiome altering the nutrient supply of the host. In this study, we assessed the effects of common MP polymers (PET, PLA and PS) on the fitness of the soil-dwelling woodlouse Porcellio scaber and the associated gut microbiome. During an eight-weeks feeding experiment, 10 adult woodlice were exposed to food pellets containing no, 2.5% or 5% MP. No significant effects on P. scaber's fitness (survival, reproduction, growth, weight and locomotive activity) were observed, but molecular analysis of the gut microbiota revealed significantly higher bacterial abundance for individuals fed with 5%-PLA-food. Radial microsensor pH, oxygen and hydrogen concentration profiles were recorded from guts of woodlice fed with 5%-MP-food. Anoxia and acidic pH were measured for all guts. Maximum gut hydrogen (produced during fermentation) concentrations indicated significantly more production in woodlice fed with PLA-food than without MP and lowest production in woodlice fed with PET- and PS-food. These results demonstrate that woodlice are a source of hydrogen in oxic environments, suggest a positive effect of PLA on the activity of the gut microbiome of P. scaber likely due to hydrolysis and fermentation of lactic acid, while PET and PS show negative effects on the fermentation. Future analysis of the short-chain organic acid composition, the microbial diversity and active taxa in the gut will allow a better understanding. Despite the effects on the microbiota, P. scaber is widely unaffected reflecting the resilience of adult woodlice.