The Sky Blues had the bye over the weekend but finished on top with 67 points, five ahead of second-placed Midland-Guildford.
The decider will take place at South Perth’s home ground Richardson Park from 10am.
Midland-Guildford confirmed its spot in the decider after defeating Subiaco Floreat by 10 wickets at Lilac Hill on Sunday.
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Caitlin Couch (5-12) and Sheldyn Cooper (4-12) did the damage as the Swans bowled out Subi for 63 in an astonishing display.
The Swans eased to the target inside 13 overs, led by Taneale Peschel (29 not out) and Chloe Piparo (26 not out).
Midland-Guildford needed to win with third-placed Melville also triumphing in their match on Sunday, thrashing University by 176 runs at Tompkins Park.