Left winger Alex Kelsall scored a spectacular try against Letchworth
Diss will join Norfolk rivals North Walsham in the new-look National League after winning London Two North.
The Mackenders side were confirmed as champions after beating Letchworth 33-12 in the final game of the season.
Diss head coach Gavin Brock told BBC Radio Norfolk he was delighted they would be challenging for the honour of Norfolk's top club next season.
"It'll be interesting with some of our lads who used to play there, so they'll be two interesting games," he said.
"It'll also be a nice local one for us, because some of the travelling will be to Portsmouth, Jersey, places like that," he added.
After a season in which they had spent many months in second place behind Staines, Brock was delighted things had come together in the last few weeks of the campaign.
"We had a very good start, whichever league you're in, if you make a good start winning becomes a habit, we had a little sticky patch before Christmas, and we came through that.
"Going down to Staines and beating them, and Staines slipping up on a couple of occasions, it all came good at the end of the season as the boys have peaked," he said.
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