Proteins and polysaccharides are the two major ingredients and often used together in processed food. In aqueous solution, protein–polysaccharide complexation leads to form either in one- or two- phase systems. In recent years, pea protein has received increased attention because of its nutritional value and low price. However, the complexation between pea protein and polysaccharide at concentrated levels and their application have not yet been reported. As such, the overall objectives of this project were: i) to study the phase behaviors of concentrated solutions of pea protein isolate (PPI)–pectin mixtures; ii) to illustrate the microstructure and quantify physicochemical properties of PPI–pectin complexes; and iii) to explore applications of PPI–pectin complexes. We demonstrated that the state diagram could explicitly identify critical phase transition pH values (pHs) (pHc, pHφ1 and pHφ2) of soluble complexes and complex coacervates at concentrated biopolymer system. The pHopt could be recognized at the net charge neutrality or the highest storage modulus of the biopolymer mixture. As the mixing ratio increased from 1:1 to 20:1, the pHs shifted towards higher pH in PPI–pectin mixture. The higher overall charge density of low methoxyl pectin (LMP) favors complex coacervates formation over a wider pH range as compared with high methoxyl pectin (HMP). Electrostatic interaction and hydrogen bonding were the two major bonds attributed to the complexation between PPI and pectin. Additionally, smooth inner pore surfaces with homogeneous large pore size of PPI‒sugar beet pectin (SBP) microstructure could be formed at the late stage of coacervates when the environmental pH is near the pHφ2 compared to coacervates formed at pHopt. In terms of application, the formation of PPI–pectin soluble complexes could shift minimum protein percentage solubility towards more acidic pH and slightly increase the thermal denaturation temperature of PPI. For application of hempseed oil (HSO) microencapsulation by means of complex coacervates, the spray-dried microcapsules prepared at late stage of complex coacervates (pH 2.5) had higher drying efficiency and encapsulation efficiency than that coacervates formed at pH 3.5. However, the oxidative stability of HSO microcapsules using the PPISBP coacervates fabricated at pH 2.5 was significantly shortened.