We investigated the role of long non-coding RNA DLEU1 (deleted in lymphocytic leukemia 1) in the progression of papillary thyroid carcinoma (PTC). DLEU1 levels were higher in PTC cell lines (BHP5-16, TPC-1,8505C, and SW1736) and patient tissues (n=54) than in a human thyroid follicular epithelial cell line (Nthy-ori3-1) or adjacent normal thyroid tissues. High DLEU1 expression correlated positively with lymph node metastasis and advanced clinical stages in PTC patients. Bioinformatics, dual luciferase reporter, and RNA pulldown assays confirmed that DLEU1 directly binds to miR-421. Moreover, bioinformatics and dual luciferase reporter assays showed that miR-421 directly binds to the 3’untranslated region of the rho-related coiled-coil kinase 1 (ROCK1) in TPC-1 cells. PTC patient tissues and cell lines showed high ROCK1 mRNA and protein levels as well as low miR-421 levels. CCK-8, flow cytometry, wound healing, and Transwell invasion assays demonstrated that DLEU1 silencing decreases TPC-1 cell proliferation, survival and progression, but they can be rescued by miR-421 knockdown or ROCK1 overexpression. DLEU1 knockdown in TPC-1 cells decreased in vivo xenograft tumor size and weight compared to controls in nude mice. These findings demonstrate that DLEU1 promotes PTC progression by sponging miR-421 and increasing ROCK1 expression.