The adult's wingspan of the Willow Beauty is 40–48 mm.
Running over the fore- and hindwings in a semicircle are the two blackish bands commonly found in Ennominae, but they are broken and somewhat indistinct in this species.
A tell-tale characteristic is a single bulge (outward curve) in the forewing part of the outer blackish bands.
The antennae of the males (first 4 photos) are strongly feathered, while those of females (last photos) are almost smooth.

Caught in my self made moth trap.

1st photo: In my yard, Hilversum, The Netherlands, 30 August 2020
2nd photo: In my yard, Hilversum, The Netherlands, 16 September 2018
3rd photo: In my yard, Hilversum, The Netherlands, 16 September 2023
4th photo: In my yard, Hilversum, The Netherlands, 9 September 2023
5th photo: In my yard, Hilversum, The Netherlands, 30 May 2014