Electrochemically self-doped TiO2 nanotube arrays (known as reduced TiO2 nanotube arrays, r-TiO2 NTAs) are currently drawing great attention as emerging and promising materials for energy and environmental applications as they exhibit highly enhanced electrochemical properties, such as good capacitive properties and electro- and photocatalytic activity when compared to pristine TiO2 NTAs. Such enhanced properties are attributed to the introduction of trivalent titanium (Ti(III)) as a self-dopant in the lattice of pristine TiO2 NTAs through simple electrochemical reduction. However, in spite of the great interest in, and potential of this material, there is no comprehensive review on the synthesis and applications of r-TiO2 NTAs. Therefore, in this review, we critically and briefly review r-TiO2 NTAs in terms of the electrochemical self-doping mechanism, their functional features, and various applications including photolysis, dyesensitized solar cells (DSSCs), biomedical coatings and drug delivery. In addition, to better understanding r-TiO2 NTAs, pristine TiO2 NTAs are briefly introduced. Furthermore, this review proposes future research directions with major challenges to be overcome for the successful development of r-TiO2 NTAs, such as to standardize matrices for performance evaluation, to confirm the organic degradation performance as anode, and to improve mechanical stability.