John Douglas Arnold (born 1974) is an American billionaire hedge fund manager, philanthropist, and former energy trader. He began his career at Enron Corporation in 1995 as a trader, quickly rising through the ranks due to his analytical skills and innovative trading strategies. After Enron's collapse in 2001, Arnold joined Centaurus Energy, where he honed his expertise in natural gas trading. In 2002, he co-founded Centaurus Advisors, a highly successful hedge fund that managed billions in assets and generated substantial returns, allowing Arnold to retire at the age of 38 in 2012 with an estimated net worth exceeding $3 billion. He became one of the youngest self-made billionaires and has been involved in various investments, including as an investor in Eva Moskowitz's Success Academy Charter Schools.

Post-retirement, Arnold shifted focus to philanthropy, co-founding Arnold Ventures in 2016 with his wife, Laura Arnold, a former attorney and fellow philanthropist. The organization, based in Houston, Texas, is a major player in bipartisan policy advocacy, committing over $700 million to causes such as criminal justice reform, education, public health, and fiscal policy. Arnold Ventures supports initiatives like reducing incarceration rates, improving evidence-based policing, and expanding access to quality education through charter schools and research. Arnold has been a prominent donor to political causes, contributing to both Democratic and Republican candidates and super PACs, reflecting his nonpartisan approach to influence policy change. His work has drawn praise for data-driven philanthropy but also criticism for its aggressive lobbying tactics in areas like drug policy and sentencing reform.

Arnold's influence extends through his board roles and advisory positions in organizations focused on systemic reform. He resides in Houston with his family and continues to shape networks of influence in finance, politics, and social policy, emphasizing long-term societal impact over short-term gains.