This study investigated the effects of the invasive polychaete Marenzelleria viridis (Marenzelleria) on the biogeochemistry of sandy marine sediments. A 27-day microcosm experiment was conducted to evaluate net microbial reactions, fluxes and porewater profiles of key solutes under the presence of Marenzelleria in two sediment types; one containing autochthonous organic matter (0.4 %) and another slightly amended with seagrass detritus (Ruppia maritima; 0.5 %). Fluxes of TCO2 and O2 were two- to three-fold increased in the presence of Marenzelleria and were higher in amended (256 mmol m−2 d−1) than in non-amended sediments (117 mmol m−2 d−1). Marenzelleria stimulated carbon degradation and sulfate reduction in both sediment treatments by influencing porewater chemistry and DOC availability. Marenzelleria flushed out inhibitory porewater metabolites (i.e. TCO2, TH2S and NH4 +) while replenishing SO4 2−. Furthermore, sulfate consumption was significantly higher in Marenzelleria sediment (79–87 mmol m−2 d−1) than in defaunated controls (36–50 mmol m−2 d−1), which indicated a 75–92 % lower DOC availability in the former. DOC excreted by Marenzelleria may have sustained 17–32 % of SO4 2− reduction. Lower C:S ratios (1.6–1.7) in Marenzelleria compared to defaunated sediments (2.4–2.8) suggest basic differences in carbon mineralization pathways, with e.g. H2 as a possible substrate for sulfate reduction in the former treatment and higher fermentative TCO2 formation in the latter. No significant stimulatory effect of Marenzelleria was evident for microbial NH4 + production. Total budgets revealed that Marenzelleria increased C mineralization by 50 % in amended sediment, compared to only 9 % in non-amended sediment. This study suggests that the presence of Marenzelleria may lead to a marked stimulation of sulfate reduction and degradation of refractory organic matter. Marenzelleria invasion may therefore have widespread consequences for microbial pathways and organic matter processing in sandy marine sediments.