Wire arc additive manufacturing (WAAM) is a metal 3D printing technique that is well recognised in the construction sector for its high efficiency, cost-effectiveness and flexibility in build scales. However, there remains a lack of fundamental data on the structural performance of WAAM elements, especially regarding their fatigue behaviour. A comprehensive experimental study into the fatigue behaviour of WAAM steel plates has therefore been undertaken and is reported herein. Following geometric, mechanical and microstructural characterisation, a series of WAAM coupons was tested under uniaxial high-cycle fatigue loading. A total of 75 fatigue tests on both as-built and machined coupons, covering various stress ranges and stress ratios, have been conducted. The fatigue test results were analysed using constant life diagrams (CLDs) and S-N (stress-life) diagrams. The CLDs revealed that the fatigue strength of the as-built WAAM steel was relatively insensitive to the different stress ratios. The S-N diagrams showed that the surface undulations resulted in a reduction of about 35% in the fatigue endurance limit for the as-built WAAM material relative to the machined material, and a reduction of about 60% in the fatigue life under the same load level. The as-built and machined WAAM coupons were shown to exhibit similar fatigue behaviour to conventional steel butt welds and structural steel S355, respectively. Preliminary S-N curves were also proposed for the WAAM steel.