SOX11 expression has been recently shown to be useful in the diagnosis of mantle cell lymphoma (MCL), including cyclin D1-negative MCL with typical morphology. We evaluated SOX11 expression pattern in B-cell non-Hodgkin lymphoma (B-NHL) subtypes to confirm specificity and used it as a feature to identify the first reported cases of cyclin D1-negative blastoid MCL. SOX11 expression was evaluated by immunohistochemistry in 140 cases of mature B-NHL, including 4 cases of suspected blastoid MCL that lacked cyclin D1 expression and 8 cases of CD5-positive diffuse large B-cell lymphoma (DLBL). In addition, 5 cases of B or T lymphoblastic lymphoma were included. Nuclear expression of SOX11 was found in cyclin D1-positive MCL (30/30, 100%) and in a case of cyclin D1-negative MCL with typical morphology. SOX11 was also expressed in Burkitt lymphoma (1/5, 20%) and lymphoblastic lymphoma (2/3 T-LBLs, 2/2 B-LBLs, overall 4/5, 80%), whereas all cases of DLBL (including CD5 DLBL) and other small B-NHL were negative. The 4 suspected cases of blastoid MCL were also SOX11. These cases had a complex karyotype that included 12p abnormalities. We confirmed prior reports that stated that SOX11 nuclear expression was a specific marker for MCL, including cyclin D1-negative MCL with typical morphology. To our knowledge, this is the first report regarding its use in identifying cases of cyclin D1-negative blastoid MCL. Routine use of SOX11 in cases of suspected CD5 DLBL might help identify additional cases of cyclin D1-negative blastoid MCL.