Gabrielle Union & Dwyane Wade -- Welcome To Our Wedding ... Now Forget You Saw Anything!!

Dwyane Wade and Gabrielle Union are sealing the deal on their super secret wedding with a contract that demands everyone remain totally silent about what they see that day... and violators will be dealt with harshly!

We've obtained a copy of the confidentiality agreement drawn up for D-Wade and Gabby -- we're told relatives, guests, vendors, caterers ... in short, everybody ... will have to sign it before setting foot in the wedding.

TMZ Sports broke the story ... guests have already been told not to bring cell phones or cameras to the wedding this Saturday in South Florida.

The doc goes even further ... warning that ANYTHING seen or heard at the wedding is confidential information that must be kept secret -- including any personal info or financial details.

According to the agreement, violators will pay a hefty cash fine for unspecified damages.