ABSTRACT T cell activation after contact with an antigen‐presenting cell depends on the regulated assembly of the T cell receptor signaling complex, which involves the polarized assembly of a stable, raft‐like macrodomain surrounding engaged T cell receptors. Here we show that the preformed reggie/flotillin caps present in resting T cells act as priming platforms for macrodomain assembly. Preformed reggie‐1/flotillin‐2 caps are exceptionally stable, as shown by fluorescence recovery after photobleaching (FRAP). Upon T cell stimulation, signaling molecules are recruited to the stable reggie/flotillin caps. Importantly, a trans‐negative reggie‐1/flotillin‐2 deletion mutant, which interferes with assembly of the preformed reggie/flotillin cap, impairs raft polarization and macrodomain formation after T cell activation. Accordingly, expression of the trans‐negative reggie‐1 mutant leads to the incorrect positioning of the guanine nucleotide exchange factor Vav, resulting in defects in cytoskeletal reorganization. Thus, the preformed reggie/flotillin caps are stable priming platforms for the assembly of multiprotein complexes controlling actin reorganization during T cell activation.