King's beat Otago Boys' High School by two wickets in the regional final at Tonga Park.
The South Dunedin school will now play at the national final in Lincoln in December.
It is only the second time King's has qualified in the 24-year history of the Gillette Cup.
In 2000, when the national final featured only four teams, King's was led by a rising star called Brendon McCullum, named player of the tournament.
Otago Boys' has represented the Otago association at the last three finals.
It won back-to-back titles in 1992 and 1993, and provided three of the first four most valuable players - Rob Lawson, Chris Finch and Grant Billcliff.
The tournament is a production line of talent. As well as McCullum, Matthew Bell, Jamie How, BJ Watling, Corey Anderson, Tim Southee, Kane Williamson and Adam Milne have all been named MVP then gone on to play international cricket.
The highest score in the event's history is 136, posted by Simon Hickey, who was named player of the tournament in 2010 and 2011 and is now the starting first five-eighth for the Blues.
Otago centre Michael Collins and Crusaders first five Tyler Bleyendaal are other rugby players of note to have starred at the Gillette Cup in recent summers.
Yesterday's regional final was all about the bowlers.
Otago Boys' batted first and was dismissed for a paltry 56, only Josh Finnie (11) reaching double figures.
King's bowlers Joel Meade (three for seven), Kurt Johnston (three for 17), Liam Trainor (two for 15) and Eamon O'Brien (two for 15) shared the wickets.
Any thoughts King's might have had of a comfortable victory were soon dashed.
The school slumped to 17 for four, 33 for six and 48 for eight. Elliot Love grafted 41 balls for his unbeaten 10 to guide his team to victory.
Finnie had the magnificent figures of four for 13 off 10 overs for Otago Boys', and he was also to the fore in the previous day's semifinal.
Otago Boys' posted an imposing 250 for nine - Finnie whacked 62 off 46 balls, and Blake Buttar-Scurr had 52 not out off 37 - and rolled Southland Boys' High School for 108. Finnie took three wickets.
King's beat John McGlashan College by six wickets.
McGlashan was dismissed for 162 (Hamish Hendry 43; Meade four for 19, Trainor three for 27), and King's chased down the target through Johnston (51 not out) and Meade (46).
