The Duke and Duchess of Sussex had box seats Monday at Crypto.com Arena -- having come on down from the Santa Barbara area to catch the big playoff game against the Grizzlies  ... which King James and co. secured in down-to-the-wire dramatic fashion.

Their presence came as a surprise to many when the TNT broadcast showed them in their private suite up in the stands -- and they seemed to be enjoying themselves ... flashing grins.

Later, they were caught on the Kiss Cam ... with their image being plastered on the giant jumbotron. The couple didn't really lock lips or anything, though ... they just giggled until the camera cut to someone else. However, it does seem PH might've considered a smooch.

Pretty neat to see 'em in such a public setting like this ... considering we hardly ever do. Harry and Meg tend to stick to the 805 area code, more or less, and hardly venture to the heart of L.A. And if they do ... they come privately, as they're hardly snapped by paps here.

Like we mentioned, they were in good (famous) company yesterday -- other stars in the building included Kim Kardashian, Adam Sandler, Timothy Olyphant, Flea and more.

We'll see more of Harry next weekend for his dad's coronation -- where Harry's expected to be front and center for the actual crowning ceremony on Saturday. Meghan won't attend.