Titania nanoparticles (P25) are successfully chemically bonded with graphdiyne (GD) nanosheets by a facile hydrothermal treatment, to form a novel nanocomposite photocatalyst. The as-prepared P25-GD nanocomposite exhibits higher photocatalytic activity for degrading methylene blue under UV irradiation than not only P25 and P25-carbon nanotube composite but also the current well-known P25-graphene composite photocatalysts. Moreover, P25-GD also shows considerable visible-light-driven photocatalytic activity, since the formation of chemical bonds between P25 and GD effectively decreases the bandgap of P25 and extends its absorbable light range. The photocatalytic activity of P25-GD can be adjusted by changing the content of GD in composites and the optimized value is about 0.6 wt%. Such a nanocomposite photocatalyst might find potential application in a wide range of fields including air purification and waste water treatment.