John Marsden is Professor of Addictions Psychology and Head of Department, Addictions, at the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King's College London. 

John Marsden is a clinical research psychologist with a research focus on the study of behavioural and pharmacological therapies for substance use disorders. He has held professional appointments in the addictions field since 1987. He has research interests in treatment systems research, psychometrics, and psychiatric epidemiology.

John is the European editor of Addiction. He was a member of the Advisory Council on the Misuse of drugs (2005-2009) and a founding member of the Independent Scientific Committee on Drugs.