A flexible secondary battery includes: an electrode stack assembly including a first electrode plate, a second electrode plate, and a separator between the first electrode plate and the second electrode plate; a first electrode tab electrically connected to the first electrode plate; and a second electrode tab electrically connected to the second electrode plate. One end of the first electrode tab and one end of the second electrode tab are disposed inside the electrode stack assembly and are stacked together with the first electrode plate, the second electrode plate, and the separator to form the electrode stack assembly. A first welding part is formed between, and binds, at least one of i) a portion of the first electrode plate and a portion of the first electrode tab and ii) a portion of the second electrode plate and a portion of the second electrode tab.