To provide a support structure for a cargo tank, a floating structure, and a method for supporting a cargo tank reducing the number and placement positions of support members, shortening the time required for construction work, and reducing the construction cost.

The support structure for a cargo tank 2 includes: a plurality of first support parts 3 distributed across a base 2a of the cargo tank 2; a second support part 4 formed in a continuous manner along the longitudinal direction of the floating structure 1 and disposed on the base 2a of the cargo tank 2; and a locking part 5 formed along the longitudinal direction of the second support part 4 on the base 11a side of a containing unit 11 and lockable with the second support part 4. The support structure is configured so that the vertical load Fz of the cargo tank 2 can be supported at least by the first support parts 3 and the horizontal load (first horizontal load Fy) of the cargo tank 2 can be supported at least by the second support part 4 and the locking part 5.