Medieval lex mercatoria refers to the customary commercial law developed by merchants to govern cross-border trade, operating alongside and sometimes independently of territorial legal systems. This paper compares that historical form of autonomous ordering with contemporary blockchain governance. Both create institutional frameworks that facilitate exchange among diverse actors and provide mechanisms that function, to varying degrees, outside traditional state authority. The key difference lies in how rules are generated and enforced: medieval merchant law relied on flexible norms interpreted by merchant courts and other human adjudicators, whereas blockchain systems seek to reduce ambiguity by encoding rules ex ante in smart contracts and automating enforcement. Decentralized decision-making and emerging forms of on-chain adjudication further reimagine dispute resolution without centralized judicial power. The central claim is that both represent polycentric legal orders whose significance ultimately depends on how they interact with, complement, or challenge formal governmental institutions.