Osteopontin (OPN), also known as secreted phosphoprotein 1 (SPP1), is a multifunctional protein that plays significant roles in various biological processes. OPN is associated with numerous inflammatory conditions, including infections, allergic responses, autoimmunity, and tissue damage. It is expressed by many cell types in the peripheral immune system and has various functions that can be either beneficial or detrimental. OPN is involved in bone metabolism and homeostasis, playing a role in neuron-mediated and endocrine-regulated bone mass. It is secreted by osteoblasts and other cells in the bone marrow and is important for the proliferation, migration, and adhesion of bone-related cells such as bone marrow mesenchymal stem cells, hematopoietic stem cells, osteoclasts, and osteoblasts. It has been implicated in the occurrence and development of several bone-related diseases, such as osteoporosis, rheumatoid arthritis, and osteosarcoma. It is also linked to cardiovascular diseases, cancer, diabetes, and kidney stone diseases. Moreover, OPN promotes tumor cell growth and induces anti-apoptotic signaling through its activation of CD44 and integrin heterodimers. It enhances tumor cell mobility, invasiveness, and epithelial-mesenchymal transition, which are crucial for dissemination and metastasis.