The present invention provides a bicycle battery holder, a bicycle battery, and a retaining member for the bicycle battery, which can improve the availability. The bicycle battery holder is configured to be mounted to a bicycle. The bicycle battery holder includes a first support portion and a second support portion. The first support portion is configured to support a battery at a first supporting position. The second support portion is configured to support the battery at a second supporting position, along with the first support portion. At least one of the first supporting position and the second supporting position is adjustably mounted.