Aluminum-ion batteries (AIBs) are considered as a promising alternative to traditional rechargeable batteries due to their high theoretical capacity, low cost, and multivalent-ion/multi-electron transfer, but are hindered from commercialization by the lack of suitable cathode materials. Herein, an aluminum-selenium (Al-Se) battery that operates at room temperature with high energy efficiency is reported. This Al-Se battery exhibits high selenium utilization with a discharge capacity of 607 mAh g−1, a reduced overpotential, and high volumetric capacity for over 100 cycles. Furthermore, ex-situ spectroscopic and microscopic investigations of the Se cathodes indicated the existence of an aluminum selenide component as the discharge product. Additionally, in-situ transmission electron microscopy provided not only insights into understanding the Al-Se electrochemistry, but also a real-time characterization technique to evaluate the electrochemistry of AIBs in general.