This study aimed to assess the ability of the activPAL monitor (commonly used for measuring Sedentary Behaviour (SB), sitting or reclining with low energy expenditure while awake) to measure moderate to vigorous physical activity (MVPA), by assessing its agreement with the concurrent measurement by ActiGraph monitor (commonly used for measuring MVPA) to identify if a single monitor could be used to measure both MVPA and SB. A convenience sample of 24 adults (79% female; aged 23-60) wore an ActiGraph GT3X+ and an activPAL3 concurrently for one day during free-living activities. Time spent in MVPA was calculated as an outcome measure using published methods (ActiGraph, n=6; activPAL n=4). Agreement was assessed between pairs of outcomes using the Bland & Altman method. Participants engaged in between 60 and 145 minutes of MVPA. The activPAL method summing time walking with a cadence ≥100 steps/min underestimated MVPA compared with the ActiGraph but had the lowest aggregate bias (-16 minutes). Other activPAL methods, based on acceleration counts and the embedded MET algorithm, overestimated MVPA compared to the ActiGraph. The study was limited by the lack of activPAL acceleration count methods developed for adults. With the recommended methods, the activPAL could be suitable for use as a single monitor to measure both SB and MVPA.