We propose and demonstrate an optical minimum-shift-keying (MSK) transmitter using a single-quad Mach–Zehnder (MZ) in-phase and quadrature modulator, where four sub-Mach–Zehnder modulators are monolithically integrated on a main MZ superstructure. Using the proposed transmitter, a MSK signal at bit rate of B could be successfully generated by orthogonally superposing two low-speed $\left(B/2\right)$ carrier-suppressed return-to-zero differential phase-shift-keying streams with a relative time offset, $1/B$ . MSK signals at bit rate of 20 and 80 Gbits/s were successfully generated using the proposed MSK transmitter. A continuous changed phase trajectory of the obtained MSK was confirmed using a complex spectrum analyzer. The filtering tolerance of the MSK was also characterized using a bandwidth-variable tunable filter.