Hoshino, who was in a three-way tie for the lead going into the final day, carded a four-under-par 68 to finish on 14-under overall, a shot ahead of Coussaud and two ahead of Scotland's Scott Jamieson in third place.
The Japanese golfer made a nervy start, mixing two bogeys with three birdies on the front nine but rediscovered his touch after the turn, with a birdie on the 10th and consecutive birdies on the 16th and 17th holes to claim the $425,000 top prize.
"I'm honoured to win at this wonderful tournament," Hoshino said. "Of course I was nervous but I just tried to keep enjoying it."
Hoshino, who becomes the fourth Japanese to win a DP World Tour title after Isao Aoki, Hideki Matsuyama and Ryo Hisatsune, was tied with Denmark's Rasmus Hojgaard and Coussaud on 10-under heading into the final round.
Hojgaard dropped off the pace early, with bogeys on the first and fourth and another after the turn seeing him sign for a one-over-par 73 in his closing round and end in a three-way tie for sixth place.
Jamieson was the day's big mover, charging up 15 places in the standings with six birdies and an eagle, offset by a single bogey to sign for a seven-under par 65 and 12-under overall.