Objectives: To better understand the role of irisin in humans, we examined the effects of irisin in human primary adipocytes and fresh human subcutaneous white adipose tissue (scWAT). Methods: Human primary adipocytes derived from 25 female donors' fresh scWAT were used to examine the effects of irisin on browning and mitochondrial respiration and preadipocytes were used to examine the effects of irisin on adipogenesis and osteogenesis. Cultured fragments of scWAT were used for investigating signal transduction pathways as well as uncoupling protein 1 (UCP1). Individual responses to irisin in scWAT were correlated with basal expression levels of brown/beige genes. Results: Irisin up-regulated the expression of browning-associated genes and UCP1 protein in both cultured primary mature adipocytes and fresh adipose tissues. It also significantly increased thermogenesis at 5 nM by elevating cellular energy metabolism (OCR & ECAR).Treating human scWAT with irisin increased UCP1 expression by activating the ERK and P38 MAPK signaling. Blocking either pathway with specific inhibitors abolished irisin-induced UCP1 upregulation. In contrast, UCP1 in human brown adipose tissue (BAT) was insensitive to irisin. Basal levels of brown/beige and FNDC5 genes correlated positively with the browning response of scWAT to irisin. In addition, irisin significantly inhibited adipogenic differentiation, but promoted osteogenic differentiation. Conclusions: Irisin promotes "browning" of mature adipocytes and scWAT and increases cellular thermogenesis of adipocytes, but inhibits adipogenesis and promotes osteogenesis during lineage-specific differentiation. Our findings provide valuable insights into for further exploring the therapeutic use of irisin in obesity and exercise-associated bone formation.