The Janssen COVID-19 vaccine or Johnson & Johnson COVID-19 vaccine is a COVID-19 vaccine that was developed by Janssen Vaccines in Leiden, Netherlands, and its Belgian parent company Janssen Pharmaceuticals, subsidiary of American company Johnson & Johnson.

It is a viral vector vaccine based on a human adenovirus that has been modified to contain the gene for making the spike protein of the SARS-CoV-2 virus that causes COVID-19. The body's immune system responds to this spike protein to produce antibodies. The vaccine requires only one dose and does not need to be stored frozen.

Clinical trials for the vaccine were started in June 2020, with Phase III involving around 43,000 people. On 29 January 2021, Janssen announced that 28 days after a completed vaccination, the vaccine was 66% effective in a one-dose regimen in preventing symptomatic COVID-19, with an 85% efficacy in preventing severe COVID-19, and 100% efficacy in preventing hospitalization or death caused by the disease.

The vaccine has been granted an emergency use authorization by the US Food and Drug Administration and a conditional marketing authorization by the European Medicines Agency (EMA) and the UK Medicines and Healthcare products Regulatory Agency.