*NSYNC diehards are convinced a reunion is afoot after spotting a few members in the Big Apple -- and TMZ knows the band will be front and center during the show.

Justin Timberlake, Joey Fatone and Lance Bass were all photographed in and around NYC over the past 48 hours -- JT hit up the US Open, Joey was spotted roaming the streets/posing with fans and Lance got snapped grabbing drinks in Manhattan.

In addition to these sightings ... JC Chasez was pictured riding in a private jet -- through a recent social media post from his manager -- although it's unclear where exactly he was flying to.

The one person who has yet to be shot in public is Chris Kirkpatrick ... but he did post a 9/11 tribute Monday, and threw up a NYFD firetruck. Unclear if he took it himself though.

Sources with direct knowledge tell TMZ ... the group will be presenting during the show Tuesday at the Prudential Center in New Jersey. We do not know if they will also perform.

Of course, another clue of a reunion ... a track that's been teased through Justin's latest 'Trolls' movie -- which has the first letter of their band name and a catchy whistling tune ... although the full song has yet to be released/named.

The guys haven't performed together since the 2013 VMAs -- so a reunion now would be timely ... a 10-year anniversary. They haven't released new music since the early 2000s.

Lord knows their day-ones have been craving a reunion for a long, long time now.

The big award show is only hours away, so we'll see if the iconic boy band is ready to join forces. A lot has changed since they last rocked a major stage -- here's hoping they still got the juice!