Inside Scarlett Johansson and Colin Jost's Private Engagement Celebration

Scarlett Johansson actually said "Yes" to Colin Jost last week, and the couple had a private celebration at a glitzy restaurant in upstate NY ... where they could not hide their happy news.

Sources close to the situation tell TMZ ... Scarlett and the 'SNL' vet hit up Blue Hill Stone Barns last Thursday in Pocantico Hills, NY -- about 30 miles outside NYC. We're told they arrived around 10 PM ... no entourage, just the 2 of them.

It's unclear if Colin proposed on the grounds. The restaurant -- featured on "Chef's Table" -- sits on a huge farm, and it's possible Colin might have taken ScarJo to a private area to pop the question. We do know she was sporting a huge shiny ring in the dining area, and they were extremely happy -- calling friends and family to share the news.

We're told they were VERY affectionate throughout the night ... she sat on his lap and witnesses tell us it was crystal clear they were in celebration mode. The owner also treated them like true VIPs ... taking them to the kitchen where they ate one of their courses.

We're told they stayed at the restaurant til around 1 AM. It's interesting Colin proposed on Thursday and they had such a late celebration. This weekend was the 'SNL' season finale ... so BIG week for Jost, who we now know was secretly engaged when he went on the air Saturday night.

As we reported ... Scarlett and Colin announced their engagement on Sunday after 2 years of dating. This will be Scarlett's third marriage and Colin's first.