Geminivirus replication enhancer (REn) proteins dramatically increase the accumulation of viral DNA species by an unknown mechanism. In this study, we present evidence implicating SINAC1, a new member of the NAC domain protein family from tomato (Solanum lycopersicum), in Tomato leaf curl virus (TLCV) REn function. We isolated SINAC1 using yeast (Saccharomyces cerevisiae) two-hybrid technology and TLCV REn as bait, and confirmed the interaction between these proteins in vitro. TLCV induces SINAC1 expression specifically in infected cells, and this upregulation requires REn. In a transient TLCV replication system, overexpression of SINAC1 resulted in a substantial increase in viral DNA accumulation. SINAC1 colocalized with REn to the nucleus and activated transcription of a reporter gene in yeast, suggesting that in healthy cells it functions as a transcription factor. Together, these results imply that SINAC1 plays an important role in the process by which REn enhances TLCV replication.