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The ocean plays a key role in modulating the climate\ud of the Earth system (ES). At the present time it is also\ud a major sink both for the carbon dioxide (CO2) released\ud by human activities and for the excess heat driven by the\ud resulting atmospheric greenhouse effect. Understanding the\ud ocean’s role in these processes is critical for model projections\ud of future change and its potential impacts on human\ud societies. A necessary first step in assessing the credibility of\ud such future projections is an evaluation of their performance\ud against the present state of the ocean. Here we use a range\ud of observational fields to validate the physical and biogeochemical\ud performance of the ocean component of UKESM1,\ud a new Earth system model (ESM) for CMIP6 built upon\ud the HadGEM3-GC3.1 physical climate model. Analysis focuses\ud on the realism of the ocean’s physical state and circulation,\ud its key elemental cycles, and its marine productivity.\ud UKESM1 generally performs well across a broad spectrum\ud of properties, but it exhibits a number of notable biases.\ud Physically, these include a global warm bias inherited from\ud model spin-up, excess northern sea ice but insufficient southern\ud sea ice and sluggish interior circulation. Biogeochemical\ud biases found include shallow remineralization of sinking\ud organic matter, excessive iron stress in regions such as\ud the equatorial Pacific, and generally lower surface alkalinity\ud that results in decreased surface and interior dissolved inorganic\ud carbon (DIC) concentrations. The mechanisms driving\ud these biases are explored to identify consequences for the behaviour\ud of UKESM1 under future climate change scenarios\ud and avenues for model improvement. Finally, across key biogeochemical\ud properties, UKESM1 improves in performance\ud relative to its CMIP5 precursor and performs well alongside\ud its fellow members of the CMIP6 ensemble.