Recent studies have demonstrated an important role for circulating serotonin in regulating bone mass in rodents. In addition, patients treated with selective serotonin reuptake inhibitors (SSRIs) have reduced areal bone mineral density (aBMD). However, the potential physiologic role of serotonin in regulating bone mass in humans remains unclear. Thus we measured serum serotonin levels in a population‐based sample of 275 women and related these to total‐body and spine aBMD assessed by dual‐energy X‐ray absorptiometry, femur neck total and trabecular volumetric BMD (vBMD) and vertebral trabecular vBMD assessed by quantitative computed tomography (QCT), and bone microstructural parameters at the distal radius assessed by high‐resolution peripheral QCT (HRpQCT). Serotonin levels were inversely associated with body and spine aBMD (age‐adjusted R = −0.17 and −0.16, P < .01, respectively) and with femur neck total and trabecular vBMD (age‐adjusted R = −0.17 and −0.25, P < .01 and < .001, respectively) but not lumbar spine vBMD. Bone volume/tissue volume, trabecular number, and trabecular thickness at the radius were inversely associated with serotonin levels (age‐adjusted R = −0.16, −0.16, and −0.14, P < .05, respectively). Serotonin levels also were inversely associated with body mass index (BMI; age‐adjusted R = −0.23, P < .001). Multivariable models showed that serotonin levels remained significant negative predictors of femur neck total and trabecular vBMD, as well as trabecular thickness at the radius, after adjusting for age and BMI. Collectively, our data provide support for a physiologic role for circulating serotonin in regulating bone mass in humans. © 2010 American Society for Bone and Mineral Research