Tertiary lymphoid structures (TLSs) are ectopic lymphoid aggregates that can develop within or adjacent to tumors, but protocols that can accurately identify and characterize TLSs are lacking. Here, we present a protocol for the in situ interrogation and characterization of TLSs in human and murine tissue sections using Opal™-tyramide signal amplification multiplex immunohistochemistry. This protocol enables simultaneous detection of up to 7 markers (6 antigens and a DAPI counterstain). We also describe a grading system to identify immature and mature TLSs.