The basic functional requirement of a classroom environment suggests a safe, delightful and relaxing place for the learners’ physical and psychological well-being and vigor such that there will be an aligned expression of satisfaction with the thermal environment. This study assessed the operational thermal comfort of naturally ventilated one-side-window oriented classroom building in Abeokuta, Ogun state, Nigeria using objective and subjective research approaches. The objective research technique involved the use of standardized measurement devices in the assessment of associated environmental factors which were relative humidity, airflow rate, ambient and mean radiant temperatures. The human factor aspect of thermal comfort was subjectively assessed using a questionnaire structured on ASHRAE scale. The measurements were taken at the height of 130 cm at the time intervals of 7:50 – 8:30 am (morning session) and 2:20 – 3:00 pm (Afternoon session) from the month of March 2016 through to February 2017, Mondays to Fridays, excluding days of any form of holidays. The descriptive statistics of the environmental factors data obtained showed that the indoor environment of the assessed classrooms was warm and stale with an average ambient temperature range of 29.37 - 30.08 ℃, mean radiant temperature 29.24 - 31.28 ℃, relative humidity 67.81 - 68.99 % and wind speed 0.022 - 0.037 m/s. The multiple linear regression analysis gave an R-value of .943 with .889 chances that the average ambient temperature of the classrooms will be affected by time-invariants and microclimate variables which was an indication of good level of prediction. The human factors of the thermal comfort observed a variation between the perceived and the preferred thermal sensation with respect to the time of the day and seasons which was unsatisfactory for the teaching and learning process.