Heterostructured TiO2 nanoparticles/nanotube arrays (NPs/NTAs) are produced from as-anodized amorphous TiO2 nanotube arrays in water at a temperature as low as 90 °C. The phase and morphology transformation of the as-anodized amorphous TiO2 nanotube in water can be attributed to a water-induced dissolution and recrystallization mechanism in which the as-anodized amorphous TiO2 NTAs are gradually self-sacrificed, and then spontaneously morphing into the composite NPs/NTAs structure consisting of anatase NPs and thinner amorphous NTs. The composite can be further crystallized into anatase TiO2 NPs/NTAs consisting of anatase NT and anatase NPs by annealing in air at 450 °C for 3 hours. The composite anatase TiO2 NPs/NTAs have a surface area that is 1.4 times larger than that of the anatase TiO2 NTAs and possess enhanced photocatalytic activity in the photodecomposition of organic pollutants and water splitting. The photodecomposition rate of the organic pollutant rhodamine B by the anatase TiO2 NPs/NTAs photocatalyst is two times higher than that by the annealed anatase TiO2 NTAs. The enhanced photocatalytic activity of the hererostructured TiO2 NPs/NTAs arises from the large surface area of the TiO2 NPs and superior electron transport in anatase TiO2 NT. The in situ hydrothermal conversion of the microstructure from amorphous TiO2 NTAs into hererostructured TiO2 NPs/NTAs in water is very simple thereby enabling the design and fabrication of highly photoactive one-dimensional TiO2-based functional materials applicable to photocatalysis and solar energy conversion.