Introduction Cachexia is common in malignant mesothelioma (MM); half of patients have malnutrition and low skeletal muscle mass. Malnourished patients have worse quality of life (QoL). Weight loss is strongly associated with poor survival. Anamorelin is an oral ghrelin receptor agonist that improves appetite, body weight and QoL in advanced cancer. The aim of this study is to examine the efficacy of anamorelin in improving appendicular skeletal muscle mass (ASM) and patient-reported outcomes in patients with MM with cachexia. Methods and analysis A single-centre, phase II, randomised, placebo-controlled cross-over pilot study with 28-day treatment periods and 3-day washout. Forty patients will be randomised. Primary outcome is change in ASM relative to height measured by dual energy X-ray absorptiometry at end of period 1. Secondary outcomes include cancer-specific and cachexia-related QoL, objective physical activity, dietary intake and adverse events. Eligible patients will have confirmed MM, Eastern Cooperative Oncology Group 0-2, expected survival >3 months and cachexia (defined as >5% weight loss in 6 months or body mass index 2%). Ethics and dissemination Ethical approval has been granted. Results will be reported in peer-reviewed publications. Trial registration number Australian New Zealand Clinical Trials Registry (U1111-1240-6828).