Both men, aged 54 and 28, were standing on shore near the Djursunda bridge outside of Ramsdal when one of them suddenly fell in the water.
"I'd seen these two men standing on shore fishing. Suddenly one of them was lying in the water, and just after that the other one was in too, probably to try to pull up the first guy," eyewitness Åke Karlsson told the local Norrköpings Tidningar newspaper.
Karlsson approached in his boat in a response to calls for help from the two men, but it was too late.
"I realised I couldn't do anything but call (emergency number) 112," he said.
With the help of a neighbour, Karlsson eventually managed to pull one of the men up on shore and attempted to resuscitate him, but to no avail.
An hour later, rescue divers located the body of the second man.
Both men are said to be from nearby Östergötland County and there are no suspicions of foul play.
"This was a tragic accident," Magnus Holmström, duty officer with the Östgöta police, to the TT news agency.