Glamorgan set up a YB40 final date at Lord's with either Nottinghamshire or Somerset after Jim Allenby inspired them to victory over holders Hampshire at the Ageas Bowl.
All-rounder Allenby produced an inspired display, scoring an unbeaten 74 as the underdogs posted 234 in their 40 overs.
He then took 2/18 in his eight over spell, as the Welsh county restricted Hampshire to 203/8 to secure a 31-run win and a place in a first Lord's final in 13 years.
Allenby's fellow Australian Michael Hogan took 4/51 and in doing so became the competition's leading wicket taker with 26.
Glamorgan will have to wait until Monday to discover their opponents for the season finale at the Home of Cricket on Saturday 21 September, with Notts and Somerset meeting in a day/night encounter at Trent Bridge.
Tickets for the YB40 Final are available to buy online now.