Is res judicata a rule of substantive law or a rule of procedure?

The doctrine seeks to prevent multiplicity of litigation, preserve the authority of judicial decisions, and promote finality in legal proceedings. In essence, once a competent court has adjudicated a matter conclusively, it cannot be reopened in a new suit.

In Satyadhyan Ghosal v. Deorajin Debi, AIR 1960 SC 941, the Supreme Court held that:

"The principle of res judicata is based on the need of giving finality to judicial decisions. What it says is that once a res is judicata, it shall not be adjudged again."

The Court clarified that res judicata is a rule of procedure but one that serves a very important public policy function—ensuring that there is an end to litigation.

Similarly, in Daryao v. State of U.P., AIR 1961 SC 1457, the Supreme Court emphasised that the rule is founded on considerations of public policy and justice:

"The rule of res judicata is not technical; it is a fundamental principle that finality should attach to the binding decisions of courts of competent jurisdiction."

Although res judicata is procedural in nature, its consequences can affect substantive rights—for instance, by barring a party from pursuing a claim they might otherwise be entitled to. However, the rule itself does not define or determine legal rights, but rather restricts the re-litigation of already adjudicated issues. Therefore, it is rightly considered a procedural safeguard grounded in public policy and judicial economy.

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