Matthew Morrison, Girlfriend Dress as Vintage Justin Timberlake, Britney Spears

Justin Timberlake is a newly married man (to Jessica Biel), and his ex Britney Spears is betrothed to another (Jason Trawick) -- but Matthew Morrison and girlfriend couldn't help but reminisce about the former power couple.

At the Glee star's third annual Halloween party/birthday party (he turns 33 on Tuesday Oct. 30), held Sunday at Lure nightclub in Hollywood, Morrison and Renee Puente paid tribute to Timberlake and Spears during their legendary early aughts romance.

Curly-haired Morrison (who bears a passing resemblance to Timberlake, 31) channeled the "SexyBack" singer in his *NSYNC days: Wrapping up his hair in a yellow bandana, wearing tinted sunglasses, a blousy white shirt (with another bandana tied around his arm), a spiky hooded vest, leather, fingerless gloves, baggy jeans, sneakers -- and even a hands-free microphone!. Not to be outdone, Morrison's love Puente summoned Spears' hottest Video Music Awards look of all time: Her 2001 "Slave 4 U" getup, complete with blonde wig, tiny green top and blue hot pants, gold boots -- finished off with costume jewelry charms and even a fake snake!

At the bash (sponsored by Monster High and Audi), Morrison made sure to pose for cheesy boy band photos -- and even posed with Timberlake's former bandmates, JC Chasez (dressed as a beekeeper) and JC Chasez (in a clown costume), among the revelers at the party.

Stealing many romantic moments together during the bash, Morrison and Puente looked more in love than ever, an observer tells Us. Other A-list attendees included Chord Overstreet (as Brad Pitt!), Shay Mitchell fellow Gleeks Harry Shum, Diana Agron and Chris Colfer plus Colton Haynes, Christina Milian and others.