Salt tolerance of crops is becoming more and more important, owing to the constant increase of salinity in arid and semi-arid regions. Broomcorn millet (Panicum miliaceum L.), generally considered tolerant to salinity, can be an alternative crop for salt affected areas. To assess genotypic variation for vegetative-stage salinity tolerance, 195 broomcorn millet accessions from a core collection were evaluated for germination percentage, shoot length, and root length during germination in 8 mL of deionized water (control) or 8 mL of a 120 mmol L− 1 salt solution (treatment). Six genotypes with different levels of salt tolerance were selected based on the growth parameters and ion concentrations in plant at the seedling stage and used for confirmation of the initial salinity response. Substantial variation for salinity tolerance was found on the basis of salt damage index [(germination percentage under control − germination percentage under salinity) / germination percentage under control × 100, SDI] and 39 accessions exhibited strong salt tolerance with SDI lower than 20%. The salt tolerance performance of the genotypes was generally consistent across experiments. In the seedling growth study, seedling number, root length and belowground biomass were adversely affected (showing more than 70%, 50%, and 32% reduction, respectively) in sensitive genotypes compared to tolerant genotypes (35%, 31%, and 3% reduction, respectively) under 160 mmol L− 1 NaCl treatment. In general, whole-plant salinity tolerance was associated with increased Na+ concentration and Na+/K+ ratio, and salt-tolerant genotypes often had higher root and lower shoot Na+ concentration than sensitive ones. Na+ concentration in root was closely related to salt tolerance and may be considered as a selection criterion for screening salt tolerance of broomcorn millet at the seedling or vegetative stages.