It looks like Samsung's next foldable will support an S Pen stylus, according to US Federal Communication Commission filings for the S Pen Pro.

The S Pen Pro appears to be a bigger version of the S Pen with Bluetooth connectivity. The FCC filings for the pen, which are now public, reveal that it will work with the yet-to-be-announced Galaxy Z Fold 3. It will apparently also support the S21 Ultra, the Note 20 lineup, and more. The full list of supported devices is in this image from the filings:

The Galaxy Z Fold 3 will support the S Pen Pro, according to FCC filings photo 2
FCC

The stylus will likely release this year. A recent render shared by leaker Evan Blass showed what appears to be the Galaxy Z Fold 3 with a new “Fold Edition” S Pen next to it. It's assumed the Z Fold 3 will come with the Fold Edition stylus in the box. If so, perhaps the S Pen Pro will be sold separately.

Samsung has been teasing it might introduce S Pen support for its foldable. During last year’s Unpacked event, Samsung’s TM Roh said he's “happy to hear” its customers want the S Pen to work with the Fold. Then, in a blog post from December, he said Samsung planned to bring the Galaxy Note’s “most well-loved features” to other devices - an obvious nod to stylus support, as it's a key feature of the Note line. Plus, in January, Roh said Samsung will “expand the S Pen experience across additional device categories in the future".

FCC documents usually immediately precede product announcements. Given Blass also leaked that Samsung's next Unpacked event is set for 11 August 2021, we could hear more about the Galaxy Z Fold 3 and S Pen support then. The Z Flip 3, two new Galaxy Watches, and new Galaxy Buds are also expected.