Abstract Yellow mealworm larvae (YML) are considered a sustainable nutrient source for food and feed applications due to their high nutrient contents while transforming low-grade bioresources. However, limited data exist on whether the mineral composition of YML could be improved via changes in their dietary compositions. This study examined the nutritional profile and composition of the larval gut microbiome of YML reared on the novel marine bioresource, brown seaweed Saccharina latissima . Dietary inclusion (50% dry matter, DM) of S. latissima in the feed significantly increased the YML mineral content, particularly P (approx. 27%), Na (approx. 120%), K (approx. 49%), Ca (approx. 35%), Mg (approx. 35%), and I (approx. 161-fold) compared to a control diet (wheat bran). Heavy metals As (up to 1.4 mg/kg DM), Cd (up to 0.23 mg/kg DM), Pb (up to 0.03 mg/kg DM), and Hg (up to 0.007 mg/kg DM) were also found to be higher in YML exposed to the seaweed-based diet than those fed control diet. Firmicutes and Proteobacteria dominated the YML gut microbiome, but a seaweed-based feed uniquely modulated the larval gut microbial community structure. In particular, dietary S. latissima promoted certain beneficial bacterial species, such as Lactobacillus fuchuensis and Lactobacillus graminis , while suppressing Lactococcus taiwanensis and Staphylococcus succinus in the YML gut. Our study indicates that including brown seaweed as a feed ingredient for YML could be an effective dietary strategy that can enrich the mineral content of YML and favourably modulate the larval gut microbiome structure without compromising larval protein and fat contents.