Nanosized SnO2 particles (∼2 nm in diameter) are embedded in ordered mesoporous CMK-8 carbon with unique three-dimensional interconnected pore channels and used as a sodium-ion battery (NIB) anode. Due to the CMK-8 confinement effects, the growth of SnO2 is suppressed during synthesis, leading to high material electroactivity. The CMK-8 not only serves as an electronic conducting pathway, but also creates interpenetrating tunnels, which guarantee electrolyte accessibility and thus Na+ transport throughout the electrode. Moreover, the change in the SnO2 volume during sodiation/desodiation can be accommodated by the CMK-8 framework. With a high tap density of ∼1000 mg cm-3 (vs. ∼800 mg cm-3 for the conventional NIB anode, hard carbon), the SnO2/CMK-8 anode shows a high reversible capacity of 800 mA h g-1 and excellent rate capability, delivering 330 mA h g-1 in ∼10 min. The electrode charge storage mechanism is examined using synchrotron X-ray diffraction. We confirm that CMK-8 incorporation can effectively promote the SnO2-Sn conversion reaction and Sn-Na alloying reaction, which are known to be thermodynamically/kinetically difficult, increasing the electrode charge-discharge performance.