Background <p>Plant-based and pescatarian diets are generally considered healthier alternatives to meat-based diets; however, their specific effects on mortality risk among individuals with clinical obesity remain unclear. This study aims to investigate the associations between different dietary patterns and the risks of all-cause and premature mortality in patients with clinical obesity.</p> Methods <p>A nationwide cohort study using UK Biobank data included 154,986 participants with clinical obesity. Dietary patterns among these individuals were categorized as regular meat-eaters, occasional meat-eaters, pescatarians, vegetarians, and vegans. Accelerated failure time (AFT) models were used to evaluate the associations between dietary patterns and all-cause and premature mortality. Interaction analyses, subgroup analyses, and sensitivity analyses were further conducted to assess the robustness of the findings.</p> Results <p>Among individuals with clinical obesity, the all-cause mortality rates for regular meat-eaters, occasional meat-eaters, pescatarians, and vegetarian/vegans were 12.75%, 13.01%, 7.58%, and 8.15%, respectively. The corresponding rates of premature mortality were 4.31%, 3.95%, 3.02%, and 4.00%. In fully adjusted AFT models, compared to regular meat-eaters, pescatarians had significantly longer time to all-cause mortality (Time Ratio [TR]: 1.14, 95% CI: 1.04–1.26) and premature mortality (TR: 1.30, 95% CI: 1.01–1.69). Subgroup analysis revealed a significant interaction between dietary pattern and sex for premature mortality (<i>p</i> for interaction = 0.01), with the protective effect of the pescatarian diet being more pronounced in men.</p> Conclusions <p>Among individuals with clinical obesity, a pescatarian diet was associated with better survival outcomes, whereas vegetarian and vegan diets did not show similar benefits. This finding provides epidemiological evidence for future research and dietary counselling in populations with clinical obesity.</p>