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MicroRNAs (miRNAs) are small, single-stranded non-coding RNAs that play essential roles in regulating key cellular processes, and are highly conserved across species. miRNA are known to regulate immune cells and play an important role in effective CD8+ T cell (CTL) responses to viral infection and tumors. To understand the role of miRNA in CTL responses we performed microarray miRNA expression profiling and identified unique in vivo expression patterns of miRNAs in naïve and effector anti-viral CTL. In particular, miR-155, which we and others previously have demonstrated to regulate CTL responses, was significantly upregulated in effector CTL. We show that further increasing the levels of miR-155 by retroviral overexpression significantly augments anti-viral effector CTL and skews memory CD8+ T cells towards an effector memory phenotype. MiR-155 overexpression resulted in enhanced T-bet expression, which is known to be required for effector CTL and to promote effector memory cell formation. MiR-155 overexpression-induced T-bet expression was required for miR-155 mediated increases in effector CTL. Our studies have revealed an important and unexpected link between miR-155 and T-bet transcription factor in the context of in vivo CTL responses.