Defence and Security Equipment International (DSEI, formerly stylized DSEi) is a biennial defence and security trade exhibition which serves as a forum between governments, national armed forces, industry, and academics, held at ExCeL London.

DSEI 2019 comprised approximately 1,700 exhibitors from 58 countries, and DSEI 2025 is scheduled for 9–12 September 2025.

In 1999, the UK's Labour Government (First Blair ministry) privatised the joint British Army and Royal Navy Exhibitions which had taken place since 1976. Exhibition company, Spearhead, launched DSEI (then known as Defence and Systems Equipment International) and the first event took place at Chertsey, Surrey.

In 2001, the exhibition moved to its current location at the ExCeL exhibition centre in London Docklands. DSEI rebranded in 2009 to Defence and Security Equipment International.

In April 2008, Clarion Events acquired DSEI.^{[citation needed]}

In 2007 two exhibitors were expelled by the then organisers, Reed Exhibitions, after distributing brochures advertising leg irons.

In 2011 DSEI ejected two exhibitors promoting cluster munitions which are banned by the UK.

In 2021 Amnesty International reported a brochure listing "waist chains and cuffs with leg cuffs" by a company at the event.