The Regional Greenhouse Gas Initiative (RGGI, pronounced "Reggie") is the first mandatory market-based program in the United States to reduce greenhouse gas emissions. RGGI is a cooperative effort among the states of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Virginia to cap and reduce carbon dioxide (CO2) emissions from the power sector. RGGI compliance obligations apply to fossil-fueled power plants 25MW and larger within the ten-state region. As of 2021, Pennsylvania is pending RGGI membership with an anticipated start in early 2022, and North Carolina is currently considering joining RGGI.

RGGI establishes a regional cap on the amount of CO2 pollution that power plants can emit by issuing a limited number of tradable CO2 allowances. Each allowance represents an authorization for a regulated power plant to emit one short ton of CO2. Individual CO2 budget trading programs in each RGGI state together create a regional market for CO2 allowances.

The RGGI states distribute over 90 percent of allowances through quarterly auctions. These allowance auctions generate proceeds, which participating states are able to invest in strategic energy and consumer benefit programs. Programs funded through RGGI have included energy efficiency, clean and renewable energy, greenhouse gas abatement, and direct bill assistance.

An initial milestone program's development occurred in 2005, when seven states signed a Memorandum of Understanding (MOU) announcing an agreement to implement RGGI. The RGGI states then established individual CO2 budget trading programs, based on the RGGI Model Rule. The first pre-compliance RGGI auction took place in September 2008, and the program became effective on January 1, 2009. The RGGI program is currently in its fourth three-year compliance period, which began January 1, 2018.