Hannah Sampson top-scored with 52 and then took 3/5 from her four overs to steer the Stingrays to a comfortable win over Kensington, which was unbeaten in the preliminary rounds.
Southern District batted first in the grand final and was in trouble at 4/47 after Eliza Doddridge took three early wickets.
But Sampson’s half-century and then handy later contributions from Rachel Andrew (25) and Kylie Rattray (23 not out) gave the Stingrays a competitive total of 6/128.
Sampson then took the new ball and removed Kensington’s top order as the Browns slipped to 4/38 in the tenth over.
Sarah Lowe (32) and Katelyn Pope (33 no) gave their team some hope but Kensington finished on 7/106.
Southern District also won the Second Grade T20 grand final, defeating Glenelg after Febi Mansell took 6/5.