SnO₂ is a promising material for both Li-ion and Na-ion batteries owing to its high theoretical capacities. Unfortunately, the electrochemical performance of SnO₂ is unsatisfactory because of the large volume change that occurs during cycling, low electronic conductivity of inactive oxide matrix, and poor kinetics, which are particularly severe in Na-ion batteries. Herein, ultra-fine SnO₂ nanocrystals anchored on a unique three-dimensional (3D) porous reduced graphene oxide (rGO) matrix are described as promising bifunctional electrodes for Li-ion and Na-ion batteries with excellent rate capability and long cycle life. Ultra-fine SnO₂ nanocrystals of size ~6 nm are well-coordinated to the graphene sheets that comprise the 3D macro-porous structure. Notably, superior rate capability was obtained up to 3 C (1/n C is a measure of the rate that allows the cell to be charged/discharged in n h) for both batteries. In situ X-ray diffractometry measurements during lithiation (or sodiation) and delithiation (or desodiation) were combined with various electrochemical techniques to reveal the real-time phase evolution. This critical information was linked with the internal resistance, ion diffusivity (D_Li⁺ and D_Na⁺), and the unique structure of the composite electrode materials to explain their excellent electrochemical performance. The improved capacity and superior rate capabilities demonstrated in this work can be ascribed to the enhanced transport kinetics of both electrons and ions within the electrode structure because of the well-interconnected, 3D macro-porous rGO matrix. The porous rGO matrix appears to play a more important role in sodium-ion batteries (SIBs), where the larger mass/radius of Na-ions are marked concerns.