Background: Gastric cancer is one of the leading causes of cancer-related deaths worldwide. Whilst surgery is the mainstay of curative treatment, it is associated with significant risks. Surgical strategies for treating gastric cancer should be based on evidence from systematic reviews of well-designed randomised controlled trials. However, inconsistencies in the reporting of outcomes from these trials makes evidence synthesis unreliable. We present a protocol for an international consensus study to develop a standardised set of outcomes and measurement tools - a 'core outcome set' (COS) - to be used by all future trials examining therapeutic surgical interventions for gastric cancer. The GASTROS study aims to standardise the reporting of outcomes in gastric cancer surgery trials through an international consensus process of key stakeholders including health care professionals and patients. Methods: The first of three stages in the study will identify a 'long-list' of potentially important outcomes to be prioritised. These will be extracted from a systematic review of relevant academic literature and patient interviews. Stage 2 will comprise an eDelphi survey which will consider the views of patients, nurse specialists and surgeons to prioritise the most important outcomes. A meeting of stakeholder representatives will ratify the COS. Stage 3 will focus on identifying appropriate instruments to measure the prioritised outcomes by means of quality assessment of available measurement instruments and stakeholder consultation. Discussion: This study aims to standardise the reporting of outcomes in future trials examining therapeutic surgical interventions for gastric cancer. It is anticipated that standardisation of outcome reporting in these surgical effectiveness trials will enhance the evidence base for clinical practice. Highlighting outcomes of greatest importance to patients will ensure that their perspectives are central to research in this field.