Chris Dudley, aged 33, pulled out the ammunition, with the date 1883 stamped on it, which is thought to have been used in a Hotchkiss revolving cannon, a weapon which was invented in 1872 and used by the German authorities and capable of firing 68 rounds per minute.
He called the police and army experts were drafted in while cordons and road closures were put in place.
Mr Dudley, from Hunslet, usually searches for artefacts from the old mills in Leeds and this is the first time, since he started magnet fishing in January, that he has found anything like this - and says he wouldn't recommend it as a stress reliever.
He said: "It is the first time I have pulled out anything like that and I wouldn't recommend it. It is dangerous, potentially live and has been there an awfully long time so you don't know how stable it is. Once you find it you have to cordon the area, phone the police.
"I only went fishing this morning because I am so nervous and excited about the football and went to relax, pulled that out and that went out of the window. It is a lot older than some of the stuff that has been pulled out and it was definitely a shock."
It comes as two years ago, fellow magnet fishers hooked out a mortar bomb that was live and just a fortnight ago, a hand grenade was pulled out of the river at a similar location.
The ammunition was removed from the scene by military experts who are expected to dispose of the find.
A spokesperson for West Yorkshire Police said: "At 12.07pm police were called to a suspicious object having being pulled from the river at Crown Point Road. A cordon and road closure was put in place as a safety precaution pending the arrival of army bomb disposal experts who have safely recovered the item.
"Police would like to thank the public for their patience during this public safety operation."