It’s believed Kate Middleton owns and wears this black cashmere crew neck jumper by British brand Boden.
Due to the nature of the garment, it’s difficult to say for certain—but plenty of elements match up, so I suspect the ID is correct.
A closer look at the ribbing detail around the neckline:
The sweater retailed for $170 at bodenusa.com and £115 boden.co.uk (was £115).
The Princess wore the sweater twice in public in 2022 and once in 2023 (as far as we can tell).