The in planta association of the Hyaloperonospora arabidopsidis effector ATR1 with the cognate Arabidopsis thaliana RPP1 immune receptor activates a disease-resistance signaling pathway that inhibits pathogen growth. To define the molecular events specifying effector recognition by RPP1, we determined the crystal structure of ATR1 and assayed in planta the effects of surface polymorphisms that are critical to activating plant immunity. ATR1 adopts an elongated, all-helical, two-domain, seahorse-like structure with an overall architecture unlike any previously described fold. Structural comparisons highlight a tandemly duplicated, five-helix motif in the C-terminal domain that creates a structural framework for rapid diversification. Identification and mapping of critical recognition sites suggest that ATR1 detection by the RPP1 resistance protein is mediated by several distinct protein surfaces that allow the effectors to escape recognition through diverse surface polymorphisms. ATR1 gain-of-recognition mutants demonstrate that multiple amino acid substitutions are necessary for recognition and that surface polymorphisms exert additive effects. These results suggest that ATR1 is a modular repeat protein belonging to an ancient family of oomycete effectors that rapidly evolves to escape host detection and adopt diverse virulence functions.