Abstract The performance of a synthetic diamond X-ray detector during typical clinical beam characterisation procedures was compared to the performance of standard clinical detectors; this diamond detector used a single crystal diamond film synthesised using chemical vapour deposition as its sensitive element. Measurements were performed using 6 MV photons from a Varian 600C linear accelerator. The procedures measured the dose profile with depth along the central axis in a phantom (tissue maximum ratio) for a 10 × 10 cm 2 field, variation in dose at the isocentre with field size (output factor), and dose profile across and beyond the X-ray beam (off-axis ratio) for 10 × 10 cm 2 and 1 × 1 cm 2 fields. Tissue maximum ratio values were within 0.8% of the values from a standard ion chamber, over a depth range of 1–15 cm. Output factors were measured for field sizes from 0.6 × 0.6 cm 2 to 15 × 15 cm 2 and agreed well (<1.3% difference) with available ion chamber data for field sizes down to 3 × 3 cm 2. Off-axis measurements showed reduced penumbral width when the lateral size of the detector was reduced by holding the diamond detector in an edge-on orientation; values were comparable to those obtained using a diode detector. Overall, these results demonstrate the potential of synthetic diamond detectors for clinical beam characterisation, particularly for small beam sizes.