The 6ft 4in 23-year-old finished last season at Eastleigh, having also appeared on loan from Grimsby Town in the Vanarama National League.
He becomes Robins boss Gary Johnson's fifth new signing of the summer following Aaron Downes, Jack Munns, Kyle Storer and Amari Morgan-Smith. The length of his contract is yet to be finalised.
Johnson said he was excited to work with Pell, who he believes is yet to reach his full potential.
"I am pleased to bring Harry in because he was one of our first thoughts this summer," Johnson said.
"I’ve always felt he hasn’t quite reached the potential I believe he can reach yet.
"He is a big lad, who is strong and a good athlete. He gets about the pitch and I think he can dictate games even more than he has been.
"I am really pleased because people I’ve spoken to enjoyed playing with him, they know of him and he is exactly what I need at this club at this moment in time.
"He is full of enthusiasm. He is relocating from his home in Essex, he’s a real Essex boy and could be in that show we all know and love!
"We both feel we can enhance each other’s future. He knows this season is very important to us and we need to get back into the Football League as quickly as we can."
Pell impressed against Cheltenham for Hereford in the FA Cup second round during the 2012/13 campaign, when the Robins progressed to play Everton after a dramatic replay victory.
He has also been on the books of Charlton Athletic and Bristol Rovers.