Revisiting the origins of the Interstate Commerce Commission (ICC) created in 1887 and offering a fresh interpretation that the commission was conceived and operated as a highly court-like agency, this paper argues that its emergence triggered the judicialization of the U.S. administrative state. It has been argued that the blueprint of the ICC took after existing railroad commissions. Its proponents in Congress, however, redesigned it with judicial courts as a model after facing criticisms based on the common-law principle of the supremacy of law allowing adjudication only to judicial courts. In accordance with such an institutional scheme, both the president and judiciary promoted the commission's judicialization by appointing lawyers as its members and reviewing its decisions. By the early twentieth century, the ICC was a prototypical agency whose court-like features permeated the administrative state. This paper thus offers a corrective to the literature on the U.S. administrative state building that has come to trivialize the role of the rise of the ICC. It was, instead, a critical juncture in the emergence of the modern administrative state in which being “quasi-judicial” was the norm rather than the exception for an administrative agency.