The 75-year-old man seems to have set off up the field before the javelin had landed, witnesses suggested.
Although he was taken to the nearest hospital, it was confirmed on Monday morning that he did not survive the night.
"He was hit in the head, in the cheek, and also sustained a wound to his carotid artery," Michael Sandforth, spokesman for the Düsseldorf fire department told The Local on Sunday.
"He was bleeding very badly and lost a lot of blood.
"Our emergency doctor administered first aid and then he was taken to hospital.”
"There was a helicopter at the scene but the doctor made the decision that there was an urgent need to bring him back round so he was taken nearby to the university clinic in the emergency doctor's vehicle,” said Sandforth.
Shocked onlookers needed counselling after witnessing the incident.
The event, held every year at the Rather Waldstadion stadium, is described as Düsseldorf's most traditional athletics event, with events for both children and adults. It has been held there every year since the end of World War II.
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This year's competition included sports such as hurdles, discuss, shot-put and high jump, as well as javelin.