Abstract: A bicycle-mountable hydration system includes a first and a second bladder, each-configured to hold liquid, and a holder configured to be supported by a top bar of a bicycle. The holder has first and second outward-facing sides and first and second inward-facing sides. The holder includes a left portion that is configured to receive the first bladder at the first-inward-facing side and a right portion that is configured to receive the second bladder at the second inward-facing side and a middle portion connecting the left portion and the right portion. When the holder is supported by a top bar of a bicycle, the first-inward-facing side of the left portion faces the first-inward-facing side of the right portion. The middle portion may hang over the top bar of the bicycle to support the holder with the first bladder on an opposite side of the top bar from the second bladder. The system may be convertible to function as a hydration backpack when removed from the bicycle.