IQGAP1 binds several proteins including actin, calmodulin, E-cadherin, beta-catenin, Cdc42, Rac1, and CLIP-170. The interaction with these targets enables IQGAP1 to participate in many cellular functions varying from regulation of the cytoskeleton to gene transcription. Here we show that extracellular signal-regulated kinase (ERK) 2 binds to IQGAP1. In vitro analysis with purified proteins demonstrated a direct interaction between ERK2 and IQGAP1. Moreover, binding occurred in cells as endogenous ERK2 co-immunoprecipitated with IQGAP1 from human breast epithelial cell lysates. The association between ERK2 and IQGAP1 was independent of epidermal growth factor. The in vivo interaction has functional significance. Manipulation of intracellular IQGAP1 levels significantly reduced growth factor-stimulated ERK1 and ERK2 activity. Similarly, stimulation of ERK1 and ERK2 activity by insulin-like growth factor I was reduced when IQGAP1 levels were changed. In contrast, overexpression of an IQGAP1 construct lacking the ERK2 binding region did not interfere with activation of ERK1 and ERK2 by epidermal growth factor. Our data disclose a previously unidentified communication between IQGAP1 and the ERK pathway and imply that IQGAP1 modulates the Ras/mitogen-activated protein kinase signaling cascade.