Across Western Europe, politicians and media express concerns about whether Muslims feel part of liberal democracies and trust national parliament. Although often assumed, research is actually inconclusive on whether Islamic religiosity hampers such political trust. Current study dives into this complex association by studying over 5000 Muslims across 17 Western European countries. It makes two core contributions. First, instead of comparing Muslims to non-Muslims, we address Muslims in specific to gain understanding in how various religiosity dimensions – mosque attendance, religious identification and praying - relate to trust in national parliament. Second, we theorize and test how exclusionary conditions buffer or aggravate Islamic religiosity's impact on political trust. We theorize how formal boundaries (e.g. restrictive citizenship policies) and informal boundaries (e.g. hostile public attitudes) affect Muslims' political trust and the role of Islamic religiosity herein. After uniquely harmonizing ESS, EVS and WVS data, multilevel analyses show that Islamic religiosity can relate negatively to political trust, yet its importance is gendered and contextualized. For example, Muslim men who more often attend a mosque experience less trust, and more so in exclusionary societies, whereas we find indications that mosque attendance among women stimulates such trust in the least hostile environments. Moreover, Muslims' political trust is in general lower in societies with more hostile public attitudes towards migrants and where political participation is restricted for some. Our study has thus illustrated that Islamic religiosity matters for Muslims' political trust in Western Europe but neither in clear-cut simple ways nor being all explanatory.