We report the composition- and gate voltage-induced tuning of transport properties in chemically synthesized Bi2(Te1-xSex)3 nanoribbons. It is found that increasing Se concentration effectively suppresses the bulk carrier transport and induces semiconducting behavior in the temperature-dependent resistance of Bi2(Te1-xSex)3 nanoribbons when x is greater than ∼10%. In Bi2(Te1-xSex)3 nanoribbons with x ≈ 20%, gate voltage enables ambipolar modulation of resistance (or conductance) in samples with thicknesses around or larger than 100 nm, indicating significantly enhanced contribution in transport from the gapless surface states.