The win marked the fourth straight time the race had gone to a French horse, after Saphresa had won the last three editions of the mile contest.
Siyouma came through in the middle part of the race and finished strong to see off the challenge of Elusive Kate on the other side of the course.
Laugh Out Loud, who made the early running, settled for third, while Beauty Parlour, favourite coming into the race, failed to get going and was never in contention.
"I was worried about the ground because she is better on a softer surface. Every time she's done a bit of work on the soft she's flown," said winning trainer Francois Doumen afterwards.
"She has been improving through the year and only came to herself when she came over to run in the Falmouth. She was quite immature and hardly ran at three. She is easy to train, has an easy temperament and Gerald said he won more easily that it may have looked."