The efficient selective synthesis of furans and pyrroles has been realized by means of a palladium-catalyzed tandem addition/ring-opening/cyclization reaction of 2-(3-aryloxiran-2-yl)acetonitriles with arylboronic acids in aqueous medium for the first time. The chemoselectivity of this transformation can be predominantly governed by the steric effects of arylboronic acids: the reactions of 2-(3-aryloxiran-2-yl)acetonitriles with the less sterically hindered arylboronic acids provided furans as the sole products, while those of the sterically bulky arylboronic acids delivered pyrroles selectively. The ¹⁸O-labelled experiments revealed that the oxygen atom of furans is derived from water, and a plausible mechanism for this selective synthesis of furans and pyrroles was proposed.