The molecular mechanisms underlying spermine osteo-inductive activity on human adipose-derived stem cells (ASCs) grown in 3-dimensional (3D) cultures were investigated. Spermine belongs to the polyamine family, naturally occurring, positively charged polycations that are able to control several cellular processes. Spermine was used at a concentration close to that found in platelet-rich plasma (PRP), an autologous blood product containing growth and differentiation factors, which has recently become popular in in vitro and in vivo bone healing and engineering. Adipose tissue was surgically obtained from the hypodermis of patients undergoing hip arthroplasty. Patient age negatively affected both ASC yield and ASC ability to form 3D constructs. ASC 3D cultures were seeded in either non differentiating or chondrogenic conditions, with or without the addition of 5 μM spermine to evaluate its osteogenic potential across 1, 2, and 3 weeks of maturation. Osteogenic medium was used as a reference. The effects of the addition of spermine on molecular markers of osteogenesis, at both gene and protein level, and mineralization were evaluated. The effects of spermine were temporally defined and responsible for the progression from the early to the mature osteoblast differentiation phases. Spermine initially promoted gene and protein expression of Runx-2, an early marker of the osteoblast lineage; then, it increased β-catenin expression and activation, which led to the induction of Osterix gene expression, the mature osteoblast commitment factor. The finding that spermine induces ASC to differentiate toward mature osteoblasts supports the use of these easily accessible mesenchymal stem cells coupled with PRP for orthopedic applications.