Sussex added the Pro40 crown to the Twenty20 Cup that they won in August
Sussex made up for this week's County Championship relegation by winning their second one-day trophy of the season at New Road.
The Pro40 holders lost by 49 runs against Worcestershire at New Road.
But Somerset's narrow two-wicket defeat by Durham at Taunton meant that one-day specialists Sussex keep their title to add to August's Twenty20 Cup win.
Worcestershire's win, helped by three wickets for Gareth Batty in his last game, ensured that they finished third.
Despite the result, it was Sussex's day - just three days after being relegated in the County Championship.
But they were made to sweat for 35 minutes before learning they had won the last-ever Pro40 title.
Replying to Worcestershire's 214-6, Sussex had crucially lost three wickets for just eight runs as they lurched to 88-6.
They did recover to make a bit of a fist of it by being bowled out for 165, thanks to three sixes in Piyush Chawla's 32 from 33 balls.
But, having lost the game against Worcestershire's young, rejuvenated side, they then needed the help of Durham, who beat Somerset at Taunton, to win cricket's latest defunct competition.
Worcestershire had three players making farewell appearances.
Surrey-bound Steven Davies, who dashed straight off after the match to join the England squad in South Africa, made 33 from 22 balls.
And Batty shared six wickets with Ian Fisher (3-18 from his 5.5 overs), who is not being retained.
Sussex's bid to reach their target misfired after Rory Hamilton-Brown had taken 16 runs off Jack Shantry in the opening over.
Seam bowlers Chris Whelan and Gareth Andrew made a double breakthrough and Ed Joyce tamely smacked the last ball of Fisher's first over to mid-on.
Murray Goodwin raised hopes for a while but a serious collapse was under way when Fisher held the easiest of return chances.
Mike Yardy was unlucky to be run out by a deflection from the bowler, Daryl Mitchell. And Sussex must have thought it would not be their day when Worcestershire's fielding reached new heights.
Moeen Ali, who had earlier hit 51, his first one-day half-century of the season at New Road, ran round the deep midwicket boundary to brilliantly hold onto a chance from Chris Nash. And Andrew Hodd was stunned by Mitchell's one-handed catch at short fine leg.
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