Yes it is an Adiantum, likely Adiantum pedatum - Northern maidenhair fern.

That would be my guess too….

I just need to look under my north facing deck, the ground carpeted with lady ferns and a diminutive forest ground cover type. A fancier one had shown up as well, though don't know what became of it after having moved it somewhere.

Adiantum pedatum doesn't occur naturally on the west coast, and it's leaves don't match. The best match for your area is Adiantum x tracyi. X tracyi leaflets increase in size as they reach the base of the stem. The leaflets on your plant seem more uniform all along the stem, but x tracyi is still the closest match. Adiantum caudatum leaflets are more uniform like your plant's, but it's an Asian species that shouldn't be on the west coast. Adiantum x tracyi? It's a jewel, and a keeper.

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There are two distinct qualities to the fern in the picture that I don't quite see in any of the images when I search on Adiantum x tracy nor pedatum, and that's the curvy and branching nature of the leaf stems and the black midrib. They are striking and unusual. Most of the pictures show fairly straight and uniform leaf patterns.

Here is a good example pedatum, black midrib, but very straight stems.

Where are you located? Is it A aleuticum?

Somehow I overlooked aleuticum, and it's a perfect match. Sorry about that SeniorBallon! Adiantum aleuticum!

The parents of x tracyi are aleuticum and jordanii. That's when I noticed aleuticum and hurried back here 😳🏃‍♂️