Matthew Morrison surprised fans during a recent concert in London, by having special guest JC Chasez sing an old NSYNC song with him!

The "Glee" star performed "This I Promise You" Monday night after one of his friends proposed on stage (she said "Yes!") -- and mid-way through had original NSYNC member JC join on in.

The crowd, which reportedly included Chelsy Davy, went crazy ... but unfortauntely Justin Timberlake, Joey Fatone and Chris Kirkpatrick didn't pop up as well.

Check out the awesome video above!