St Just beat Grampound Road by 17 runs to win the WT Edwards Cup for the eighth time in nine years.
The final is contested by the winners of west Cornwall's Vinter Cup and east Cornwall's Hawkey Cup.
Batting first, Gary Thomas made 44 to help St Just to 130-8 from their 20 overs after being 35-4 at one stage.
In reply, Grampound rallied thanks to David Hoskings' 39 but were all out for 113, with man of the match Justin Stephens posting figures of 3-10.
St Just will now represent Cornwall in the ECB Club T20 area finals day in South Wales next month.