THAT woman behind the Foster Grants is none other than Meredith Vieira – who disguised herself yesterday as a birthday surprise for “Today” co-host Matt Lauer.

Vieira and Lauer share a birthday – it’s today, actually – but since they don’t work on the weekends, Vieira, who’s been off this week, pulled her surprise on Lauer yesterday by donning a black wig and sunglasses.

She blended into the crowd gathered on the “Today” show plaza, holding a sign that read “Matt, I’m Your Biggest Fan. 49 & Fabulous!” Vieira screamed out “We love you, Matt!,” catching Lauer off-guard when he approached her. Then he realized who she was.

“I saw the sign and I tapped you on the head and I thought, ‘Why is this nut job not putting the sign down?,’ ” Lauer said to Vieira, as the crowd laughed. “If she’s my biggest fan, why won’t she say hello to me?” Vieira then led the crowd in a rousing rendition of “Happy Birthday” and presented Lauer with a gift – a lamp in the shape of a woman’s leg (made famous in the 1983 movie, “A Christmas Story”).

Natalie Morales filled in this week for Vieira, who returns Monday.