The aim of this study was to design and validate an observational instrument used to analyse the in-game behaviour of the football goalkeeper (GK) in defence and attack. The validation and reliability testing processes were carried out by twelve experts through five steps. Contents validity was calculated using Aiken’s V and the kappa index was used to analyse reliability. The observational tool was divided into three different content blocks: GK’s offensive actions, GK’s defensive actions, and opponent’s actions taken before a GK’s defensive action. Only three out of the total of 24 items achieved low values that, after modifications, recorded optimal values (values higher than .90) in accordance with the Aiken’s V. Following step one, new proposals were evaluated after a pilot test. The reliability test scored optimal values (values higher than .85). An observational sheet was developed to manage the quantitative and qualitative assessments of experts, and the proposed tool was accepted as valid and reliable.