Context: Animal studies indicate that osteocalcin (OC), particularly the undercarboxylated isoform (unOC), affects insulin sensitivity and secretion, but definitive data from humans are lacking. Objective: The objectives of the study were to determine whether total OC and unOC are independently associated with insulin sensitivity and β-cell response in overweight/obese adults; whether glucose tolerance status affects these associations; and whether the associations are independent of bone formation, as reflected in procollagen type 1 amino propeptide (P1NP). Design, Setting, and Participants: This was a cross-sectional study conducted at a university research center involving 63 overweight/obese adults with normal (n = 39) or impaired fasting glucose (IFG; n = 24). Main outcome measures: Serum concentrations of total/undercarboxylated OC and P1NP were assessed by RIA; insulin sensitivity was determined by iv glucose tolerance test (S I -IVGTT), liquid meal test (S I meal), and homeostasis model assessment of insulin resistance; β-cell response to glucose [basal β-cell response to glucose; dynamic β-cell response to glucose; static β-cell response to glucose; and total β-cell response to glucose] was derived using C-peptide modeling of meal test data; and intraabdominal adipose tissue was measured using computed tomography scanning. Results: Multiple linear regression, adjusting for intraabdominal adipose tissue and P1NP, revealed that total OC was positively associated with S I -iv glucose tolerance test (P < .01) in the total sample. OC was not associated with S I meal or homeostasis model assessment of insulin resistance. In participants with IFG, unOC was positively associated with static β-cell response to glucose and total β-cell response to glucose (P < .05), independent of insulin sensitivity. Conclusions: In overweight/obese individuals, total OC may be associated with skeletal muscle but not hepatic insulin sensitivity. unOC is uniquely associated with β-cell function only in individuals with IFG. Further research is needed to probe the causal inference of these relationships and to determine whether indirect nutrient sensing pathways underlie these associations. Copyright © 2013 by The Endocrine Society.