Article III’s diversity jurisdiction provisions extend the federal judicial power to state law controversies between different states or nations and their respective citizens. When exercising diversity jurisdiction, the federal judiciary does not function in its usual role of protecting federal interests or ensuring the uniformity of federal law. Instead, federal courts operate as alternative state courts for resolving disputes between diverse parties. But federal courts often cannot act as alternative state courts because of Article III justiciability doctrines such as standing, ripeness, and mootness. These doctrines define when a federal court may act. But they do not apply to state courts. Rather, states have developed their own justiciability doctrines that substantially diverge from the federal ones. The consequence is that federal courts sitting in diversity cannot hear many claims that can be brought in state court and can hear other claims that state courts lack the power to decide. This Article argues that, instead of applying federal justiciability doctrines, federal courts should apply state justiciability doctrines to state law cases brought under diversity jurisdiction. Following state justiciability doctrines would better achieve the goals of allowing federal courts to function as alternative state courts. Moreover, following state justiciability doctrines in state law cases would not undermine the rationales underlying federal justiciability doctrines because those doctrines were developed to limit the federal judiciary’s ability to interfere with the other branches of the federal government— concerns that are inapplicable in state law disputes.