The comedian - who met Timberlake through the singer's appearances on Saturday Night Live - said that guests were required to sign the document before attending.
"I signed a contract," Fallon told Andy Cohen during an appearance on Watch What Happens Live yesterday (November 1).
Despite admitting that he couldn't go into detail about the nuptials, the 39-year-old talkshow host admitted that he found the ceremony "beautiful".
"She looked like a princess," he said of Biel. "It was just insane. Everything was beautiful.
"I'm a sucker for weddings. I'm a fun person to invite to a wedding. Because I dance, I get up and drink. But then, I weep."
This week, Timberlake's former 'N Sync bandmates said that they harboured "no hard feelings" over missing out on invites to the wedding.
"It would be a reunion rather than a wedding," said Chris Kirkpatrick of the snub. "It was [Biel's] day.
"She's amazing so he wanted to make sure it was about her."