Multiple myeloma (MM) is characterized by the expansion of malignant plasma cells in the bone marrow (BM). Most MMs display aberrant Wnt/β-catenin signaling, which drives proliferation; however, they lack oncogenic Wnt-pathway mutations, suggesting activation by autocrine Wnt ligands and/or paracrine Wnts from the BM microenvironment. Expression of the heparan sulfate proteoglycan (HSPG) syndecan-1 is a hallmark of MM. Syndecan-1 is a critical player in the complex reciprocal interaction between MM cells and their BM niche, mediating growth factor/cytokine binding and signaling by its heparan sulfate (HS) chains. Here, by means of CRISPR/Cas9-mediated knockout and doxycycline-inducible shRNA-mediated knockdown of EXT1, a critical enzyme for HS polymerization, we demonstrate that the HS-chains decorating syndecan-1 mediate aberrant Wnt-pathway activation in MM. HS-deficient MM cells exhibited a strongly decreased autocrine Wnt/β-catenin pathway activity and a reduced Wnt-pathway-dependent proliferation. In addition, we demonstrate that Wnts bind to the HS side-chains of syndecan-1 and that this binding contributes to paracrine Wnt-pathway activation through the Wnt-receptor Frizzled. Furthermore, in a HS-dependent fashion, syndecan-1 also binds osteoblast-produced R-spondin, which represses Frizzled degradation by activation of LGR4, an R-spondin receptor aberrantly expressed on MM cells. Co-stimulation with R-spondin, and its binding to HS chains decorating syndecan-1, are indispensable for optimal stimulation of Wnt-signaling in MM. Taken together, our results identify syndecan-1 as a crucial component of the Wnt-signalosome in MM cells, binding Wnts and R-spondins to promote aberrant Wnt/β-catenin signaling and cell growth, and suggest HS and its biosynthetic enzymes as potential targets for the treatment of MM.