Silicon photonics promises to address the challenges for next-generation short-reach optical interconnects. Growing bandwidth demand in hyper-scale data centers and high-performance computing motivates the development of faster and more efficient silicon photonics links. While it is challenging to raise the serial line rate, further scaling of the data rate can be realized by, for example, increasing the number of parallel fibers, increasing the number of wavelengths per fiber, and using multilevel pulse-amplitude modulation (PAM). Among these approaches, PAM has a unique advantage because it does not require extra lasers or a costly overhaul of optical fiber cablings within the existing infrastructure. Here, we demonstrate, to our knowledge, the first fully monolithically integrated silicon photonic four-level PAM (PAM-4) transmitter operating at 56 Gb/s and demonstrate error-free transmission (bit-error rate <10–12)up to 50 Gb/s without forward-error correction. The superior PAM-4 waveform is enabled by co-design and co-optimization of the silicon traveling-wave modulators with the monolithic CMOS driver circuits. Our results show that monolithic silicon photonics technology is a promising platform for future ultrahigh data rate optical interconnects.