Opal (amorphous hydrated silica) has been found within a brecciated ureilite meteorite (EET 83309) found in Antarctica. This meteorite was derived from the surface regolith of an unknown differentiated parent asteroid. The opal occurs as replacement of ureilitic olivine and suessite (Fe3Si), and as broken fragments among the clasts derived directly from the parent ureilite asteroid. It can be banded or unbanded, and could easily be mistaken for graphite in back-scattered electron images. It contains considerable amounts of FeO and in some places is devitrifying to a SiO2 phase. The textures strongly suggest that the opal formed before the meteorite was delivered to Earth, as in places it is surrounded by a terrestrial weathering rim. Thus it reflects the presence of water on or near the surface of the parent asteroid. Ureilites are well-known for being completely anhydrous, so the presence of opal implies that the water which reacted with the ureilite minerals came from another source within the Solar System. We suggest that this may have been an impact of an icy planetesimal on the surface of the ureilite parent asteroid. Oxygen isotopes in the opal range from values close to those of ureilite meteorites towards isotopically-light Antarctic meteoric water, suggesting that exchange has taken place between terrestrial water and the porous extraterrestrial opal during the time the meteorite was resident in Antarctica.