Tin chalcogenide has recently received a great deal of consideration due to its versatile structures and interesting electrical properties. Herein, we report an unprecedented crystalline Sn−Se compound, {Se 8 Cs 3 @[Sn 3 Se 13 ]}n Cs 3 Sn 3 Se 21 , featuring elemental Se 8 rings and Cs + ions intercalated between the [Sn 3 Se 13 ] n 3nlayers. Dark single crystals of Cs 3 Sn 3 Se 21 show unusual metallic lust, a narrow band gap of 1.31 eV, and an electrical conductivity of 1.24 × 10 −5 S/cm at ambient temperature. Under visible-light illumination, Cs 3 Sn 3 Se 21 generates a high photocurrent density of 1.2 μA cm −2 at a biased low potential of 0.8 V. More importantly, Cs 3 Sn 3 Se 21 can be used for the Liion battery anode material, showing excellent specific capacity of 490 mAh g −1 at a current density of 50 mA g −1 . Furthermore, the material demonstrated high stability; even after cycling at 2000 mA g −1 , the capacity can be restored to 488 mAh g −1 once the current density is recovered, confirming the superior rate performance.