In a post published on the OnePlus forum, Pei noted that he made a "difficult decision," after his association with the company for seven years.
"After nearly 7 years at OnePlus, I've made the difficult decision to say goodbye. I grew up spending tremendous amounts of time on the internet, building products and communities. Seeing that ideas in one's mind could be turned into reality and impact people's lives gave me lots of joy, and I knew early on that this was the path for me," Pei wrote on Saturday.
Recently, reports of Pei's departure had surfaced in the media ahead of the company's major launch event for OnePlus 8T.
Pei co-founded OnePlus with CEO Pete Lau in December 2013. Pei had been instrumental in designing the OnePlus smartphone line-up.
The smartphone brand last week launched its new flagship 5G smartphone, the 'OnePlus 8 T' with 120Hz Fluid AMOLED display and quad-camera set up, in the Indian market.
The OnePlus 8T will be available in two colours -- aquamarine green and lunar silver -- at Rs 45,999 (12GB RAM+ 256GB internal storage) and Rs 42,999 (8GB RAM+128GB internal storage).