Measuring changes in psychomotor activity is a potential tool in the monitoring of the course of affective states in bipolar disorder. Previous studies have been cross-sectional and only some have used objective measures. The aim was to investigate state-related differences in objectively-measured psychomotor activity in bipolar disorder. During a 12 weeks study, repeated measurements of heart rate and movement monitoring over several days were collected during different affective states from 19 outpatients with bipolar disorder. Outcomes included activity energy expenditure (AEE) and trunk acceleration (ACC). Symptoms were clinically assessed using Hamilton Depression Rating Scale (HDRS-17) and Young Mania Rating Scale (YMRS). Compared to patients in a euthymic state, patients in a manic state had significantly higher AEE. Compared to patients in a depressive state, patients in a manic state had significantly higher ACC and AEE. There was a significant diurnal variation in ACC and AEE between affective states. Finally, there was a significant correlation between the severity of manic symptoms and ACC and AEE, respectively. This first study measuring psychomotor activity during different affective states using a combined heart rate and movement sensor supports that psychomotor activity is a core symptom in bipolar disorder that is altered during affective states.