Athletics at the 1912 Summer Olympics – Men's 400 metres

It was the fifth appearance of the event, which is one of 12 athletics events to have been held at every Summer Olympics. None of the finalists from 1908 returned. The favorites were all among by the American team: 1909 and 1911 AAU champion Edward Lindberg, 1911 IC4A champion Donnell Young, and 1912 IC4A champion Charles Reidpath.[3]

Australasia, Austria, Bohemia, Japan, Portugal, Russia, and South Africa appeared in the event for the first time. The United States made its fifth appearance in the event, the only nation to compete in it at every Olympic Games to that point.

The competition consisted of three rounds. The first round had 15 heats, ranging from 1 to 6 runners. The top two runners in each heat advanced to the semifinals. The semifinal was to consist of 5 heats of 6 runners each, but one semifinal had only 5 runners because one preliminary heat had only had 1 runner. Only the top runner in each semifinal heat advanced, making a five-man final. The first two rounds were run without lanes, but an incident in the last semifinal resulted in the final being held with strings demarking lanes.[3][4]

These were the standing world and Olympic records (in seconds) prior to the 1912 Summer Olympics.

World record	Maxie Long (USA)	47.8(*)	New York, United States	29 September 1900
Olympic record	Wyndham Halswelle (GBR)	48.4(**)	London, United Kingdom	22 July 1908

(*) unofficial 440 yards (= 402.34 m)

(**) This track was 536.45 metres=​¹⁄₃ mile in circumference.

The Olympic record of 48.4 seconds, set at the previous Olympics, fell in the final. Charles Reidpath broke the record with a 48.2 second performance. Reidpath's time was ratified by the IAAF as the inaugural official world record. Hanns Braun also broke the old Olympic record and Edward Lindberg tied it, en route to silver and bronze medals, respectively.

• Bergvall, Erik (ed.) (1913). Adams-Ray, Edward (trans.). (ed.). The Official Report of the Olympic Games of Stockholm 1912. Stockholm: Wahlström & Widstrand.CS1 maint: extra text: authors list (link)
• Wudarski, Pawel (1999). "Wyniki Igrzysk Olimpijskich" (in Polish). Retrieved 27 August 2006.