The 21-year-old started the opening Guinness Premiership game of this campaign against Bristol and has since battled with Matt Powell and Ryan Powell for the starting number nine shirt.
John Brain, Worcester Warriors Director of Rugby, said: "Nick has done an excellent job for Worcester over the last two seasons and is maturing into a fine scrum half. We're delighted that he'll be with us for another two years.
"In Nick, Matt Powell and Ryan Powell we have quality and real competition for the scrum half berth going forward into next season."
Cheltenham-born Runciman, who attended Cheltenham Bournside School and Sixth Form College, has enjoyed international rugby success at England U16, U18s and U21 level.
Runciman was also a member of the RFU Junior National Academy and younger brother Will is currently on the Academy books at Sixways.
"I'm very pleased to have signed a new deal and I'm really looking forward to the future and developing my game further here," he said.
"Worcester Warriors are a great club to be a part of and I'm excited by the ambitious plans for the future, both on and off the field, and being a key part of a successful team."