CuZnSn(S, Se) (CZTSSe) thin-film solar cells are showing great promise due to using earth-abundant and non-toxic materials and tuning the bandgap through the amount of S and Se. Flexible high-efficiency CZTSSe solar cells are one of the outstanding research challenges because they currently require the use of thick glass substrates due to the high-temperature heat treatment process, and for this reason, few flexible CZTSSe solar cells have been reported. Furthermore, most researchers have used thin glass and metal substrates with little flexibility and the power conversion efficiencies (PCEs) of the solar cells made with them have been somewhat lower. To overcome these hurdles, we transferred high-efficiency CZTSSe solar cells formed on a soda-lime glass substrate to flexible substrates via an adhesive-bonding transfer method. Via this method, we were able to achieve the PCE of 5.9 to 7.1% on completely flexible substrates such as cloth, paper, and polyethylene terephthalate (PET). In particular, we were able to produce a CZTSSe solar cell on a PET substrate with a PCE of 7.1%, which is the highest among fully-flexible CZTSSe solar cells currently known to us. In addition, we deeply analyzed the PCE degradation of the flexible CZTSSe solar cell fabricated by the transfer method through a panoramic focused ion-beam image and nanoindentation. From the results of our work, we provide an insight into the possibility of making flexible high-efficiency CZTSSe solar cells using our transfer method.