Although several wide-bandgap (1.6-2.0 eV) chalcopyrites (e.g. CuGaSe2 Cu(In,Ga)S2, Cu(In,Al)Se2, (Ag,Cu)GaSe2) have been thus far studied as top cell absorbers for photoelectrochemical (PEC) water splitting tandem devices, CuGa(S,Se)2 is a wide-Eg chalcopyrite that has not yet received any attention in this regard. Thus, we present the performance of wide-bandgap chalcopyrite CuGa(S,Se)2 photocathodes as a top cell for PEC tandem water splitting. To be able to assess the PEC performance of CuGa(S,Se)2 as well as its optical transmittance, transparent conductive fluorinated tin oxide (FTO) was used as a contact. During our study though we learned that synthesizing CuGa(S,Se)2 films on an FTO substrate would degrade the optoelectronic properties of the FTO and so we present a synthesis to circumvent this problem and fabricate functioning CuGa(S,Se)2 PEC photocathodes. We then present the PEC performance of these functioning CuGa(S,Se)2 PEC photocathodes (JSAT≈10 mA/cm2) as well as measurements relevant to its performance as a top cell, such as quantum efficiency of a low-Eg Cu(In,Ga)Se2 shaded by the CuGa(S,Se)2 PEC photocathodes and sub-bandgap transmittance of the CuGa(S,Se)2 PEC photocathodes.