This research reports on the similarities and distinctive differences between informal social networks in China (Guanxi) and Korea (Yongo). Within an analytical framework derived from social capital and institutional theory, the structural forms and characteristics of both network forms are compared. Although we observe some similarities, surprisingly, the two networks show several fundamental differences. Both are society-spanning constructs, developed and maintained by reciprocal action that creates trust and trustworthiness, and serves as a major factor in network cohesion. Both networks are relatively closed or inaccessible to outsiders, with insiders able to connect other insiders to each other (i.e., internal bridging of structural holes). However, Guanxi can be characterized as being utilitarian (purpose-based), whereas Yongo in principle describes cause-based ties. Furthermore, Guanxi networks are somewhat accessible to outsiders and draw on a diverse base of ties; Yongo networks are predefined, partly by birth, and are hence homogeneous and highly exclusive. Guanxi networks can benefit from spillover effects through bridging different networks; Yongo networks often cannot, as there is antipathy, competition, and potentially hostility between certain types of networks. The results add knowledge to social network theory in general and in particular on informal social networks in East Asia.