The Mariana forearc is a unique location for exploring the role serpentinization plays in the marine Si cycle by means of Si stable isotope variations. Here, active mud volcanism transports deep, serpentinized mantle wedge material to the surface and thus offers a natural window to slab dehydration processes in dependence of changing temperature and pressure with depth. Si isotopes were measured in situby femtosecond laser ablation MC-ICPMS in serpentine within ultramafic clasts from three mud volcanoes (Yinazao, Fantangisna, and Asut Tesoru) sampled during International Ocean Discovery Program Expedition 366. To corroborate the results, serpentinization of olivine was studied in batch experiments. The Si isotope ratios show large variations between the mud volcanoes and between individual serpentine generations within a given mud volcano. Serpentine that formed early under low water/rock ratios exhibits delta Si-30 of -0.41 +/- 0.04 parts per thousand (1SD) similar to unaltered olivine which agrees well with experimental findings predicting no significant isotope fractionation during early serpentinization. In contrast, late serpentine veins formed under higher water/rock ratios span a wide range of Si isotope ratios that differ significantly between the individual mud volcanoes. With increasing distance to the trench, delta Si-30 of late veins are -0.10 +/- 0.07 parts per thousand, -1.94 +/- 0.13 parts per thousand, and -0.80 +/- 0.22 parts per thousand and -0.93 +/- 0.21 parts per thousand. These delta Si-30 values are interpreted to record the isotopic composition of the fluid source, namely subducted biogenic silica and pore fluids, clays, and altered oceanic crust that dehydrate as consequence of rising pressure and temperature with depth. We show that Si isotopes of mantle wedge serpentinites can be used as a reliable new proxy for slab dehydration processes. They may be used in paleo-forearc systems to unravel oceanic sediment and silica biomineralization evolution through geological time.