Western Australia 387 for 7 (Inglis 138, Bancroft 90, Marsh 56, W Agar 2-56) beat South Australia 206 (Hunt 50, Smith 44, Agar 5-64) by 181 runs

Promoted to open after in-form D'Arcy Short was controversially dropped, Inglis smashed 138 off 110 balls to fuel Western Australia's massive 387 for 7 after being sent in to bat.

It was Western Australia's joint-highest score in 50-over cricket as they gained a stranglehold on a record-extending 16th title in the competition.

South Australia lasted just 31.4 overs in a forlorn chase as WA successfully defended their title in a season where they won all eight matches.

"It was nice to get up to the top, the fields up...you don't get a chance to score hundreds if you're batting five or six," said Inglis, who revealed he knew about his elevation to open during a team meeting on Tuesday morning. "We'll see if I get an opportunity over there [India] but it's nice I've got the form to go back on."

Mitchell Marsh, coming in at No.4 and playing as a specialist batter, continued his impressive return after a three-month layoff from ankle surgery with a 30-ball 56 in the fastest half-century in a Marsh Cup final. After his recent ton in the Sheffield Shield match against Tasmania, Marsh hit five sixes in a showcase of his trademark power to flatten SA's flagging attack.

Hunting their first 50-over title since 2012, a nervy South Australia were out of their depth against a stacked Western Australia team fielding four players who will be part of the upcoming three-match ODI series in India.

It was a disappointing end for South Australia, who have improved considerably in the Marsh Cup and Sheffield Shield this season after a period in the wilderness.

"Obviously bitterly disappointed," SA coach Jason Gillespie said. "We know we are a lot better than what we showed out there. "We have a lot to take away and hopefully we get better."

But South Australia's fielding deteriorated from there with Inglis dropped by Henry Hunt on the boundary off spinner Ben Manenti in a pivotal moment. Having made just 26 off 42 balls to that point, Inglis suddenly clicked into gear and made South Australia pay with a belligerent assault as he needed just another 45 deliveries to notch his century.

Inglis mixed inventive strokes, including well executed reversed scoops, with sweet textbook drives to demoralise South Australia whose sloppy fielding effort also included Henry Thornton dropping Bancroft when he was on 51.

As a thunderstorm, rare in Perth this time of year, threatened to intervene, South Australia openers Hunt and Kelvin Smith provided the fireworks to notch a 50-run partnership within six overs. Having been a late replacement for Jake Carder, who injured his hamstring in the warm-up, Smith sparkled in his 50-over debut for South Australia as he pounced on unusually listless bowling from the quicks.

But he fell on 44 after tamely spooning to short midwicket as South Australia were shackled by quick Andrew Tye and spinner Agar, who claimed Hunt for 50 to trigger a slew of wickets.

It was only a matter of time before Western Australia celebrated another triumph as the powerhouse state remains on course for a treble of domestic titles for the second straight season.