Latent membrane protein 1 (LMP1) acts like a permanently activated receptor of the tumor necrosis factor (TNF)-receptor superfamily and is absolutely required for B cell immortalization by Epstein–Barr virus. Molecular and biochemical approaches demonstrated that LMP1 usurps cellular signaling pathways resulting in the induction of NF-κB and AP-1 via two C-terminal activating regions. We demonstrate here that a third region encompassing a proline rich sequence within the 33 bp repetitive stretch of LMP1's C-terminus is required for the activation of Janus kinase 3 (JAK3). The interaction of LMP1 and JAK3 leads to the enhanced tyrosine auto/transphosphorylation of JAK3 within minutes after crosslinking of a conditional NGF-R:LMP1 chimera and is a prerequisite for the activation of STAT transcription factors. These results reveal a novel activating region in the LMP1 C-terminus and identify the JAK/STAT pathway as a target of this viral integral membrane protein in B cells.