Runciman who has represented England at U16, U18 and U21 level then made the move to London Welsh in the English Championship and has made 65 league appearances for the Exiles.
He will make the move back to Kingsholm this summer and head coach Bryan Redpath was delighted with the signing.
"We know all about Nick obviously with him having started his career here as part of our Academy and then having played just up the road at Worcester," he said.
"He's developed into a very capable performer in the meantime and we're aware that Rory Lawson will probably be away for quite a bit of next season because of the Rugby World Cup and Six Nations."
"We'll therefore need options in the number nine shirt as it's a specialist position and with Nick, Dave Lewis and Dan Robson to call upon, I'm very happy with where we stand."
Runciman himself is delighted to be returning to where it all started for him:
"It'll be great to be back at Kingsholm. I obviously still know a number of the boys there and have kept tabs on how the team have been doing."
"I saw them win the LV= Cup on television recently so Heineken Cup is assured next season which is a huge bonus for the club and very exciting for me personally."
"I've really enjoyed my time at London Welsh, they're a great club and it goes without saying that I'll be doing my level best for them for the remaining games of the season.