Game theory models provide a useful framework for investigating strategies of conflict resolution in animal contests. Model predictions are based on estimates of resource holding potential (RHP) and vary in their assumptions about how opponents gather information about RHP. Models can be divided into self-assessment strategies (energetic war-of-attrition, E-WOA; cumulative assessment model, CAM) and mutual-assessment strategies (sequential assessment model, SAM). We used laboratory-staged contests between male giant Australian cuttlefish, Sepia apama,to evaluate RHP traits and to test game theory models. Mantle length was a key indicator of RHP because it predicted contest outcome, whereby individuals with larger body size were more likely to win a contest. Winners and losers did not match behaviours, ruling out the E-WOA. There was no relationship between contest outcome, duration and escalation rates, arguing against the CAM. Persistence to continue a contest was based on RHP asymmetry, rather than loser and/or winner RHP, providing support for the SAM. Motivation to fight was derived from a male’s latency to resume a contest following the introduction of a female stimulus. The latency to resume a contest was negatively related to the size of the focal male and positively related to the size of their opponent. These results infer that competing males are able to gather information concerning RHP asymmetries, providing support for mutual-assessment. Furthermore, males showed significant behavioural differences in their responses to relatively larger opponents as compared to relatively smaller opponents. This modification of behaviour as a result of information gathered from opponent cues provides compelling evidence for a mutual-assessment fighting strategy in this species. This study uses an integrative approach to test for visual RHP assessment. Our results demonstrate that male cuttlefish assess the RHP of their opponent relative to their own (i.e. SAM), providing a definitive example of mutual-assessment.