Mesenchymal stromal cells (MSCs), given their regenerative potential, are being investigated as a potential therapeutic tool for cartilage lesions. MSCs express several bioactive molecules which act in a paracrine fashion to modulate the tissue microenvironment. Yet, little is known about the divergence of these signalling molecules in different MSC populations. The present study investigated secretomes of stromal cells harvested from Hoffa's fat pad (HFPSCs), synovial membrane (SMSCs), umbilical cord (UCSCs) and cartilage (ACs) by quantitative liquid chromatography-mass spectrometry (LC-MS/MS) proteomics. Also, multiplex protein arrays and functional assays were performed to compare the constitutive immunomodulatory capabilities of different MSCs. Proteins involved in extracellular matrix degradation and inflammation, such as matrix metalloproteinases (MMPs), interleukin (IL)-17 and complement factors, were downregulated in UCSCs as compared to adult cell sources. Additionally, secretion of transforming growth factor (TGF)-β1 and prostaglandin E2 (PGE2) was enhanced in UCSC supernatants. UCSCs were superior in inhibiting peripheral blood mononuclear cell (PBMC) proliferation, migration and cytokine secretion as compared to adult stromal cells. SMSCs significantly suppressed the proliferation of PBMCs only if they were primed with pro-inflammatory cytokines. Although all cell types repressed human leukocyte antigen-DR isotype (HLA-DR) surface expression and cytokine release by activated macrophages, only UCSCs significantly blocked IL-6 and IL-12 production. Furthermore, UCSCs supernatants increased aggrecan gene expression in two-dimensional chondrocyte cultures. The data demonstrated that UCSCs displayed superior anti-inflammatory and immunosuppressive properties than stromal cells from adult tissues. This allogeneic cell source could potentially be considered as an adjuvant therapy for articular cartilage repair.