The supplementarity principle, also referred to as the supplementary principle, is one of the principles of the Kyoto Protocol. Its objective is to limit the application of the Protocol's flexibility mechanisms – emissions trading, clean development mechanism and joint implementation – and establishes that each should be supplemental to domestic action in meeting the emission reductions targets of the parties to the Protocol.

The supplementarity principle is embedded in three articles: article 6 and 17 with regards to trading, and article 12 with regards to the clean development mechanism.

Article 6.1 states that "The acquisition of emission reduction units shall be supplemental to domestic actions for the purposes of meeting commitments under Article 3". Article 17 states that "[…]Any such trading shall be supplemental to domestic actions for the purpose of meeting quantified emission limitation and reduction commitments under that article". Article 12.3.b states that "Parties included in Annex I may use the certified emission reductions accruing from such project activities to contribute to compliance with part of their quantified emission limitation and reduction commitments under Article 3[…]".

This principle has fueled a heated debate during the negotiation phase subsequent to the signature of the Kyoto Protocol, and leading up to the clarification and definition of the legal details of the provisions of the Protocol (details then formalized in the Marrakech Accords). In particular, since the letter of the Protocol does not quantify supplementarity, the European Union (EU) brought forward a proposal to that purpose; the proposal - which would practically require that domestic action represent at least 50% of a party's mitigation efforts - was though rejected during the sixth Conference of the Parties (COP6) in The Hague, Netherlands. In fact the supplementarity issue was one of the reasons why COP6 negotiations broke down (the others related to carbon sinks and non-compliance). Much of the thrust towards rejecting any formal quantification of limits to the use of the flexibility mechanisms came from the US, supported by the Umbrella Group (New Zealand, Japan, Canada, Australia, Russia, Ukraine, Norway and Iceland), which saw in any quantitative restriction to the use of the mechanisms, and of emission trading in particular, impediments to reaching cost-effective compliance.

In essence, today the supplemental provisions determine reporting obligations on parties: "Each Party included in Annex I shall provide information on how its use of the mechanisms is supplemental to domestic action, and how its domestic action thus constitutes a significant element of the effort made to meet its quantified limitation and reduction commitments", as established in the Marrakech Accords.