Parasmittina trispinosa is an encrusting bryozoan. Colonies form orange to yellowish-white sheets which have nodules distributed across the surface. Colonies may be composed of a single layer or multiple layers of autozooids. Autozooids are elongate, quadrate to oval or irregular in outline. They are 0.4-1.0 by 0.26-0.38 mm.
Parasmittina trispinosa is a cool temperate species, distributed off all British coasts. It ranges north to western Norway and the Faroe Isles, but is thought not to reach Arctic waters.
The species is able to colonise hard substrates including stones and shells. It is often found on vertical faces of large boulders. It ranges from shallow subtidal waters to the continental shelf, and is often abundant on offshore shell banks. Parasmittina trispinosa can tolerate high energy environments.