The 20-year-old has been a regular in the Black Cats youth set-up all the way up to under-23 level after breaking into the under-21 side at the end of the 2015/16 season.
The striker was sent out on loan to then-League Two side Hartlepool United in January 2017, only for an injury to cut short his time at the Victoria Ground after just four appearances.
Harrogate manager Simon Weaver told his club`s website: “He’s a number 9 type centre forward, if you get balls into him he will look after it, he’s got a little bit of aggression to him and he’s got a fierce strike behind him too.
“We needed that bit of physicality up front and we are delighted he has joined us.
“Paul Thirwell went to watch him and reported back the he liked what he saw and that he would be our type of player, he has a lot of desire and the physical attributes that make him a good all-round centre-forward.”
Nelson will be available for tomorrow’s match against Leamington at The CNG Stadium.
Meanwhile, Mitch Curry has extended his loan with the club from Middlesbrough for an additional month.
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