Tomlinsonus is an extinct genus of marrellomorph arthropod known from the Late Ordovician (Katian) Kirkfield Formation of Ontario, Canada.

Only a single partial specimen (ROMIP 66233) was recovered during fieldwork in September 2020, by independent researcher Marc Haensel.

There is also a pair of smaller projections without spines extending posteriorly from the back of the head.

[2] In a phylogenetic analysis Tomlinsonus was found a member of Mimetasteridae, as the closest known relative of Mimetaster hexagonalis, with which it shares an enlarged uniramous first appendage, with the supposed Argentinian Mimetaster species "M." florestaensis found to be more closely related to Furca.

[2] The depositional environment of the Kirkfield Formation was an open marine carbonate platform.