Increasing production of animal fibres has increased the need for a portable instrument that measures fibre diameter and associated characteristics with precision and accuracy. This research evaluated a new portable fibre tester (PFT) by measuring the diameter and related characteristics of tops and scoured fibres of wool, alpaca, and vicuña. The PFT was constructed with integrated mechanical, optical, electronic, and informatic components. Textile tops of sheep wool, alpaca fibres, and mohair goat fibres were used as standard references to calibrate the PFT and determine its accuracy and precision. The results were compared with those from a wool industry standard instrument (OFDA2000) that uses similar technology. The PFT had high accuracy (-0.01, -0.12, and -0.01 pm) for average fibre diameter (AFD) of wool, alpaca, and mohair fibres, respectively. Deviations of standard tops (ST) were within industry-accepted tolerance ranges. Standard errors, indicating precision, were low, ranging from 0.07 to 0.25 pm, 0.02 to 0.44 pm, and 0.09 to 0.024 pm, for wool, alpaca, and mohair fibre tops, respectively. The correlations of measurements of AFD from the two instruments were 0.99 for wool, alpaca, and mohair fibres, but lower for vicuña fibres (0.82). No evidence of bias was observed. Therefore, the PFT may be used as an alternative instrument for measuring fibre diameter and quantifying variation in diameter of wool, mohair, and alpaca fibres. The PFT has appeal for use in the field for practical animal selection and fleece classification based on fibre characteristics.