The two have previously kept their relationship low key, and have ignored reports that they were set to wed.
But the wedding announcement comes after it was claimed they would wed in a $20 million ceremony, after spending $3 million to fly in 500 wedding guests to Doha, Qatar, from all over the world.
A source said: 'Wissam is going to splash big when he marries Janet. It's going to be the wedding of the century.'
The announcement comes as the couple were spotted at the Giorgio Armani show on Monday during Milan Fashion Week.