The fully‐established friction factors of non‐Newtonian fluids in laminar and turbulent flow through a square duct were experimentally determined. The laminar friction factors of both purely viscous and viscoelastic fluids show good agreement with the well‐known laminar correlation for circular pipe flow,16/Re*,whereRe*is the Reynolds number proposed by Kozicki etal. for non‐Newtonian fluids in non‐circular duct flow. The established friction factors of purely viscous fluids in turbulent flow through rectangular channels agree well with the Dodge and Metzner equation for power law fluids in circular pipe flow if their generalized Reynolds number,Re′,is replaced byRe*.In the case of viscoelastic aqueous polymer solutions, it was found that the experimental asymptotic friction factors for fully established turbulent flow through a square duct are much higher than the corresponding circular tube results when compared at a fixed value of the Kozicki Reynolds number,Re*.In contrast, the turbulent rectangular duct asymptotic friction factor results for the viscoelastic fluid are in good agreement with the circular tube results when compared at a constant Reynolds number based on the apparent viscosity at the wall.