Lubricin encoded by the proteoglycan 4 (Prg4) gene is produced from superficial zone (SFZ) cells of articular cartilage and synoviocytes, which is indispensable for lubrication of joint surfaces. Loss‐of‐function of human and mouse Prg4 results in early‐onset arthropathy accompanied by lost SFZ cells and hyperplastic synovium. Here, we focused on increases in the thickness of articular cartilage in Prg4‐knockout joints and analyzed the underlying mechanisms. In the late stage of articular cartilage development, the articular cartilage was thickened at 2 to 4 weeks and the SFZ disappeared at 8 weeks in Prg4‐knockout mice. Similar changes were observed in cultured Prg4‐knockout femoral heads. Cell tracking showed that Prg4‐knockout SFZ cells at 1 week of age expanded to deep layers after 1 week. In in vitro experiments, overexpression of Prg4 lacking a mucin‐like domain suppressed differentiation of ATDC5 cells markedly, whereas pellets of Prg4‐knockout SFZ cells showed enhanced differentiation. RNA sequencing identified matrix metalloproteinase 9 (Mmp9) as the top upregulated gene by Prg4 knockout. Mmp9 expressed in the SFZ was further induced in Prg4‐knockout mice. The increased expression of Mmp9 by Prg4 knockout was canceled by IκB kinase (IKK) inhibitor treatment. Phosphorylation of Smad2 was also enhanced in Prg4‐knockout cell pellets, which was canceled by the IKK inhibitor. Expression of Mmp9 and phosphorylated Smad2 during articular cartilage development was enhanced in Prg4‐knockout joints. Lubricin contributes to homeostasis of articular cartilage by suppressing differentiation of SFZ cells, and the nuclear factor‐kappa B‐Mmp9‐TGF‐β pathway is probably responsible for the downstream action of lubricin.