Objective. The increasing interest in hadron therapy has heightened the need for accurate and reliable methods to assess radiation quality and the biological effectiveness of particles used in treatment. Microdosimetry has emerged as a key tool for this, demonstrating its potential, reliability, and suitability. In this context, solid-state microdosimeters offer technological advantages over traditional tissue-equivalent proportional counters, and recent advancements have further improved their performance and reliability. However, one critical challenge in solid-state microdosimetry is the so-called ‘border effect’, which can impact measurement accuracy. Approach. In this study, the border effect in diamond microdosimeters was thoroughly studied using ion beam induced charge analysis. The research, relying on experiments conducted at the Surrey Ion Beam Centre, developed an effective method to characterise and quantify the border effect. Geant4 Monte Carlo simulations were also employed to assess the impact of the border effect under typical proton therapy conditions. Main results. The border effect in diamond microdosimeter was characterised and studied as a function of detector thickness, particle atomic number and particle range. A border effect model was developed and validated to reproduce the border effect in Monte Carlo simulations. The results of its application to the microdosimetry of a proton beam at different depths in water showed potential significant variations of up to 40% in ¯yF and 20% in ¯yD values. Significance. The results of this work highlight the importance of accurately characterising the border effect and encouraging further research to mitigate its influence on microdosimetry measurements.