A necessary party is a real party-in-interest who is not indispensable but ought to be joined in an action. Their presence is needed if complete relief is to be accorded to those already parties or for a complete determination or settlement of the claim subject of the action.

Necessary parties have interests in the controversy that are separable from those of other parties. A final determination of the case can be made among the parties already impleaded even if a necessary party is not joined, especially for a justifiable reason.

The absence of a necessary party does not prevent the court from proceeding with the action. The judgment rendered is without prejudice to the rights of such necessary party who was not included. This is a key distinction from indispensable parties, whose absence renders subsequent actions of the court null and void.

If a necessary party is not joined in a pleading where a claim is asserted, the pleader must state the party's name, if known, and the reason for the omission. If the court finds the reason unmeritorious, it may order the pleader to join the omitted party if jurisdiction over their person can be obtained. Failure to comply with such an order without justifiable cause is deemed a waiver of the claim against that specific party.

Examples of necessary parties include co-debtors in a joint obligation, and subsequent mortgagees or lien holders in a judicial foreclosure. A possessor of land in an action for recovery of ownership may be a necessary party, while the owner is indispensable. Conversely, the owner may be a necessary party in an action to recover possession where the possessor is indispensable. Third, fourth, etc.-parties who may be liable to the defending party are also described as necessary parties. A surety who is jointly and severally liable is not an indispensable party, implying they could be a necessary party.