Backscatter communication (BackCom) is an emerging technology that enables ultra-low-power wireless transmission by reflecting ambient signals. To enhance spectral efficiency and improve communication performance, we integrate BackCom with partial nonorthogonal multiple access (NOMA). In this paper, we propose a partial NOMA-assisted backscatter communication system for device-to-device (D2D) networks, where a backscatter device (BD) reflects signals from a source to two users via distinct channels. We derive closed-form expressions for the outage probability (OP) of both users under Rayleigh fading and various decoding conditions. The analytical results provide insights into the impact of key system parameters, such as the reflection coefficient, power allocation factors, and channel conditions, on the system’s reliability. In particular, by adjusting the bandwidth allocation coefficient, we are able to compare the system performance between partial NOMA and full NOMA schemes. The findings of this study offer valuable guidance for the design of energy-efficient and spectrally efficient wireless networks utilizing BackCom and NOMA.