A multi-fuel-capable gas turbine combustor is provided which is able to sufficiently utilize various fuels having a characteristic deviating from a premixing characteristic range suitable for generating a premixed gas, even while maintaining favorable low-emission performance by premixed combustion. The gas turbine combustor includes: a main burner configured to supply a premixed gas containing a first fuel for premixing, to a first combustion region within a combustion chamber to cause premixed combustion; and a supplemental burner configured to supply a second fuel for reheating having a different composition from that of the first fuel, to a second combustion region at a location downstream of the first combustion region within the combustion chamber, to cause diffusion combustion. The first fuel has a premixing characteristic range suitable for generating a premixed gas. The second fuel has a characteristic deviating from the premixing characteristic range.