A novel magnetic iron oxide supported phenylsulfonic acid (Fe3O4@Ph-SO3H) with core-shell structure is prepared, characterized and applied as efficient nanocatalyst for green synthesis of tetrahydrobenzo[b] pyrans. The Fe3O4@Ph-SO3H was prepared via modification of magnetic iron oxide cores with 1,4-bis (triethoxysilyl)benzene (BTEB) followed by sulfonation of aromatic rings. The Fe3O4@Ph-SO3H was characterized using FTIR, TGA, PXRD, SEM, TEM, VSM and EDX techniques. This was effectively applied for synthesis of tetrahydrobenzo[b]pyrans in water as green solvent at room temperature under ultrasonic conditions. The products were obtained in high to excellent yields at short times. The recoverability, reusability and durability of this nanocatalyst were studied under applied reaction conditions. (C) 2017 Elsevier Inc. All rights reserved.