An adjustable brassiere is provided with a first adjustable buckle fastener in the middle of the interval between the two cups and second adjustable buckle fasteners in the middle of side-straps linking the two cups. Two ends of each adjustable buckle fastener are designed with two slits and a serrated cut adjacent to each inner slit. First adjustable straps extend from the two cups in the interval therebetween and are adjustable coupled to the first adjustable buckle fastener. Side-straps of the two cups include two sections from which second adjustable straps extend. The second adjustable straps are respectively adjustably coupled to the second adjustable buckle fasteners.