Stock assessments for anadromous river herring, collectively Alewife Alosa pseudoharengus and Blueback Herring A. aestivalis, lack adequate demographic information, particularly with respect to early life stages. Although sampling adult river herring is increasingly common throughout their range, currently no standardized, field-based, analytical methods exist for estimating juvenile abundance in freshwater lakes. The objective of this research was to evaluate the relative effectiveness and sampling precision of a purse seine for estimating densities of age-0 river herring in freshwater lakes. We used a purse seine to sample age-0 river herring in June–September 2015 and June–July 2016 in 16 coastal freshwater lakes in the northeastern USA. Sampling effort varied from two seine hauls to more than 50 seine hauls per lake. Catch rates were highest in June and July, and sampling precision was maximized in July. Sampling at night (versus day) in open water (versus littoral areas) was most effective for capturing newly hatched larvae and juveniles up to ca. 100 mm TL. Bootstrap simulation results indicated that sampling precision of CPUE estimates increased with sampling effort, and there was a clear threshold beyond which increased effort resulted in negligible increases in precision. The effort required to produce precise CPUE estimates, as determined by the CV, was dependent on lake size; river herring densities could be estimated with up to 10 purse-seine hauls (one-two nights) in a small lake (<50 ha) and 15–20 hauls (two-three nights) in a large lake (>50 ha). Fish collection techniques using a purse seine as described in this paper are likely to be effective for estimating recruit abundance of river herring in freshwater lakes across their range.