Bangladesh, hosting Australia in a men's ODI for the first time since 2011, posted a formidable 8-284 after being sent in at the Sher-e-Bangla National Cricket Stadium in Dhaka on Tuesday, before restricting the tourists to 9-191 from 42.2 overs when lightning and rain ended play.
It was Bangladesh's second victory in 23 ODIs against Australia, alongside their famous 2005 run chase in Cardiff.
Australia batted poorly, their fielding was shoddy and their bowling looked pedestrian compared to Bangladesh, who were spearheaded by firebrand Nahid Rana (4-41).
Nahid, mentored by Bangladesh bowling coach and former Aussie speedster Shaun Tait, troubled all the tourists with his express pace which exceeded 150km/hr.
Cameron Green (52 not out) posted his fourth ODI half-century, while Alex Carey (47) and Cooper Connolly (35) failed to capitalise on promising starts.
"Disappointing total on our part," acting Australian captain Josh Inglis said.
"We dropped four or five catches. I thought if it (Bangladesh score) was somewhere around 230-240 it was very chaseable.
"We didn't have a big partnership to get us close. Disappointing day all-round."
Australia's reply started horribly when Matt Short was bowled through the gate by Taskin Ahmed off the innings' first ball.
Then in the second over, Marnus Labuschagne (1) continued his horror patch when he was adjudged lbw to Mustafizur Rahman on a successful Bangladesh review.
Labuschagne, who tallied 24 runs at 8.00 in Australia's recent 2-1 series loss in Pakistan, is averaging 11.64 in his last 17 ODIs stretching back to 2024.
Inglis (19) nicked off to Nahid before the pair exchanged words as the Bangladeshi paceman gave the Aussie skipper a send-off.
When Carey edged Nahid to wicketkeeper Litton Das, the result was academic.
Mossadek starred with the bat in his first international in four years, capitalising on four dropped catches and two missed run-out opportunities to smash a career-best 86 not out off 70 balls.
Connolly, substitute Ollie Peake, debutant Liam Scott and Adam Zampa all spilled chances to let Mossadek off the hook.
Najmal Hossain Shanto (67) was grassed on nine by Labuschagne at second slip and made Australia pay, sharing a rapid 96-run second-wicket stand with Tanzid Hasan (54) following the early loss of Saif Hassan (5).
Scott claimed his maiden international wicket when he trapped Bangladesh captain Mehidy Hasan Miraz (3) lbw.
Nathan Ellis (3-38) was the pick of the Australian bowlers, while part-time tweaker Matt Renshaw (2-35) was useful in the middle overs.
The second ODI is in Dhaka on Thursday.
"We didn't get it right in all facets," said Carey. "Our fielding was a bit sloppy. With the bat we lost clumps of wickets Not a great way to start the series, but it is a series and we have two to go. Hopefully we can respond and play better cricket.
"We  created a number of chances but weren't able to hang on, hopefully we'll be cleaner in that and can respond on Thursday. That's the key, learn really quickly."
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