Diffuse large B cell lymphoma (DLBCL) is a common and fatal hematological malignancy. Long noncoding RNAs (lncRNAs) have emerged as crucial biomarkers and regulators in many cancers. Novel lncRNA biomarker in DLBCL needs to be investigated badly, as well as its function and molecular mechanism. To further explore, microarray analysis was performed to identify the differentially expressed lncRNAs in DLBCL tissues. To investigate the biological functions of SMAD5-AS1, we performed gain- and loss-of-function experiments in vitro and in vivo. Furthermore, bioinformatics analysis, dual-luciferase reporter assays, Argonaute 2-RNA immunoprecipitation (AGO2-RIP), RNA pull-down assay, quantitative PCR arrays, western blot assay, TOPFlash/FOPFlash reporter assay, and rescue experiments were conducted to explore the underlying mechanisms of competitive endogenous RNAs (ceRNAs). We found that SMAD5-AS1 was down-regulated in DLBCL tissues and cell lines. Functionally, SMAD5-AS1 downregulation promoted cell proliferation in vitro and in vivo, whereas SMAD5-AS1 overexpression could lead to the opposite effects in vitro and in vivo. Bioinformatics analysis and luciferase assays revealed that miR-135b-5p was a direct target of SMAD5-AS1, which was validated by dual-luciferase reporter assays, AGO2-RIP, RNA pull-down assay, and rescue experiments. Also, dual-luciferase reporter assays and rescue experiments demonstrated that miR-135b-5p targeted the adenomatous polyposis coli (APC) gene directly. SMAD5-AS1/miR-135b-5p inhibits the cell proliferation via inactivating the classic Wnt/β-catenin pathway in the form of APC dependency. Our results indicated that SMAD5-AS1 inhibits DLBCL proliferation by sponging miR-135b-5p to up-regulate APC expression and inactivate classic Wnt/β-catenin pathway, suggesting that SMAD5-AS1 may act as a potential biomarker and therapeutic target for DLBCL.