To provide an optical transmitter, optical transmission system and modulation scheme selection method capable of selecting either an optical duobinary modulation scheme or an optical DPSK modulation scheme.

The optical transmitter has: a first exclusive OR circuit which outputs as a first output signal the exclusive OR of binary logic signals input from first and second input terminals; a delay means which delays the first output signal by a predetermined amount and outputs the delayed signal to the second input terminal; a low-pass filter means which receives the first output signal as an input, converts the first output signal into a three-value signal, and outputs the three-value signal as a second output signal; and a selection means which selects either the first output signal or the second output signal on the basis of a selection signal and outputs the selected signal.