This paper introduces a new measure of informal social control of child maltreatment (henceforth ISC_CM) by neighbors. Research literature typically uses collective efficacy (Sampson, Raudenbush, & Earls, 1997) to examine neighborhood informal social control. We argue that double standards about the application of informal social control to family versus street crime requires a measure of informal social control specific to child maltreatment. We also argue that how neighbors intervene may matter as much as whether they intervene. Neighbors may engage in ISC_CM aimed at protecting the child and calming the parent, or more punitive ISC_CM aimed at deterring future abuse. We tested the relationship of both with very severe physical abuse and with abuse related child behavior problems. We used a random, 2-stage cluster design of Hanoi to collect the sample. Thirty Hanoi wards were randomly selected using probability proportional to size sampling. A simple random sample of families in each ward was then drawn using local government lists of ward residents. Based on power analysis, the target sample size was 300. Of 315 residents contacted, 293 participated, yielding a response rate of 93%. Random effects regression models (which estimate a random effect for each ward) were run in Stata11. We found that protective ISC_CM is associated with lower odds of very severe physical abuse and lower reported externalizing problems when abuse is present. Perceived collective efficacy and punitive ISC_CM is not associated with lower odds of very severe physical abuse. Implications for research, policy and practice are discussed. We conclude that further investigation of neighbor ISC_CM is needed to replicate the findings in other cultural contexts, ultimately followed by experimental manipulation of ISC_CM in a neighborhood context to examine the effects on child maltreatment. If further research corroborates the current findings, the development of neighborhood intervention programs to enhance protective ISC_CM may assist materially in reducing very severe child abuse and negative consequences stemming from such abuse.