Actin polymerization by Arp2/3 complex must be tightly regulated to promote clathrin-mediated endocytosis. Although many Arp2/3 complex activators have been identified, mechanisms for its negative regulation have remained more elusive. To address this, we analyzed the yeast arp2-7 allele, which is biochemically unique in causing unregulated actin assembly in vitro in the absence of Arp2/3 activators. We examined endocytosis in arp2-7 mutants by live-cell imaging of Sla1-GFP, a coat marker, and Abp1-RFP, which marks the later actin phase of endocytosis. Sla1-GFP and Abp1-RFP lifetimes were accelerated in arp2-7 mutants, which is opposite to actin nucleation-impaired arp2 alleles or deletions of Arp2/3 activators. We performed a screen for multicopy suppressors of arp2-7 and identified SYP1, an FCHO1 homolog, which contains F-BAR and AP-2micro homology domains. Overexpression of SYP1 in arp2-7 cells slowed Sla1-GFP lifetimes closer to wild-type cells. Further, purified Syp1 directly inhibited Las17/WASp stimulation of Arp2/3 complex-mediated actin assembly in vitro. This activity was mapped to a fragment of Syp1 located between its F-BAR and AP-2micro homology domains and depends on sequences in Las17/WASp outside of the VCA domain. Together, these data identify Syp1 as a novel negative regulator of WASp-Arp2/3 complex that helps choreograph the precise timing of actin assembly during endocytosis.