2021 European Athletics U23 Championships – Men's 400 metres

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The men's 400 metres at the 2021 European Athletics U23 Championships was held at Kadriorg Stadium in Tallinn, Estonia, from July 8–10. There were 33 competitors from 20 nations. The winning time was 45.02 seconds.

Medalists:
- Gold: Ricky Petrucciani (Switzerland)
- Silver: Jonathan Sacoor (Belgium)
- Bronze: Edoardo Scotti (Italy)

Qualification rules:
- From the heats, the first 4 in each heat plus the next 4 fastest advanced to the semifinals.
- From the semifinals, the first 2 in each heat plus the next 2 fastest advanced to the final.