The development of ultrafast ultrasound imaging brings great opportunities to improve imaging technologies such as shear wave elastography and ultrafast Doppler imaging. In ultrafast imaging, there are trade-offs among image signal-to-noise ratio (SNR), resolution, and the high frame rate. Multiplane wave (MW) imaging is proposed to solve this tradeoff by encoding multiple plane waves with positive/negative pulse polarities during one transmission event (i.e., pulse-echo event), to improve SNR in ultrafast imaging. However, it suffers from stronger reverberation clutters in B-mode images compared to standard plane wave compounding due to longer transmitted pulses. In this paper, we propose a delay-encoded harmonic imaging (DE-HI) technique to implement HI in MW compounding. It encodes the 2nd harmonic signals with 1/4 period delay calculated at the transmit center frequency during MW transmissions, rather than reversing the pulse polarity. This is because the 2nd harmonic signals cannot be encoded by pulse inversion. Received DE-HI signals can then be decoded in the frequency domain to recover the signals as in single plane wave emissions, but mainly with increased SNR at the 2nd harmonic component instead of the fundamental component. DE-HI reduces image clutters as in HI and improves image SNR as in MW. The image quality enhancement was demonstrated by an in-vivo human liver study, in which DE-HI provided enhanced contrast-to-noise ratio (CNR) and vessel identification, as compared to plane wave fundamental imaging, MW compounding, and plane wave HI without coding. The enhanced imaging quality and potential high frame rate of DE-HI made the method promising for a wide spectrum of imaging applications.