The 27-year-old from Japan, who was a runner-up twice in Australia at the start of the 2024 Race to Dubai, carded a closing 68 to finish on 14 under par at Doha Golf Club.
Hoshino went into the final round tied at the top with Rasmus Højgaard and Coussaud.
A fine approach to inside five feet at the second set up an early birdie and, after a bogey on the fourth, Hoshino made back-to-back gains on the fifth and sixth.
A wayward drive on the seventh led to another dropped shot and Hoshino returned to 12 under with a gain at the long tenth.
Jamieson was putting the finishing touches on a sensational 65 at that stage to set the clubhouse target at 12 under.
That mark held firm until Hoshino drove the 16th and made birdie, before holing a 25-footer at the next to seemingly put the result beyond doubt.
There was still time for Coussaud to add an element of tension at the last, as he made birdie from ten feet to leave his playing partner needing to convert from four feet to avoid a play-off, which he duly managed.
The victory makes Hoshino the first Japanese winner of the event, and only the fourth in DP World Tour history after Isao Aoki, Hideki Matsuyama and Ryo Hisatsune.
The result also moves him comfortably inside the top 100 on the Official World Golf Ranking.
“I'm honoured to win at this wonderful tournament,” he said. “I was second at the last two Australian tournaments so I'm so happy to finally win.
“Of course I was nervous but I just tried to keep enjoying it.”