Catecholamine-stimulated lipolysis occurs by activating adenylate cyclase and raising cAMP levels, thereby increasing protein kinase A (PKA) activity. This results in phosphorylation and modulated activity of several key lipolytic proteins. Adipose triglyceride lipase (ATGL) is the primary lipase for the initial step in triacylglycerol hydrolysis, and ATGL activity is increased during stimulated lipolysis. Here, we demonstrate that murine ATGL is phosphorylated by PKA at several serine residues in vitro and identify Ser406 as a functionally important site. ATGL null adipocytes expressing ATGL S406A (nonphosphorylatable) had reduced stimulated lipolysis. Studies in mice demonstrated increased ATGL Ser406 phosphorylation during fasting and moderate intensity exercise, conditions associated with elevated lipolytic rates. ATGL Ser404 (corresponding to murine Ser406) phosphorylation was increased by β-adrenergic stimulation but not 5′AMP-activated protein kinase activation in human subcutaneous adipose tissue explants, which correlated with lipolysis rates. Our studies suggest that β-adrenergic activation can result in PKA-mediated phosphorylation of ATGL Ser406, to moderately increase ATGL-mediated lipolysis.