Measurement of plasma osmolality (P[subscript osm]) remains popular for assessing hydration status in exercise science. However, a controlled reliability assessment of micro-osmometry using small sample volumes to measure Posm remains to be performed. This study aimed to examine the reliability of a cryoscopic micro-osmometer requiring 15-µL samples across varied environmental conditions. Thirteen young adults (26.4 ± 5.7 years) completed a 20-min steady-state cycling protocol on separate occasions in cool/dry conditions and hot/humid conditions. P[subscript osm] was assessed using fingertip capillary samples taken in duplication prior to and following 20-min acclimatization and recovery periods around the cycling bout. Absolute (typical error of measurement = 0.62-0.83 mOsmol·kg[superscript -1]; coefficient of variation = 0.9% to 1.3%) and relative (intraclass correlation coefficient = 0.60-0.85) reliability of this approach were supported. These findings suggest micro-osmometry requiring small plasma samples (15 µL) can be reliably used by practitioners to assess longitudinal changes and cross-sectional hydration status in varied environmental conditions.