Identifying bioenergy crops that can be produced successfully on marginal lands, such as those affected by salinity, reduces the pressure to produce energy crops on land that would otherwise be used to produce food crops. In this paper, the degree of salinity tolerance of “Red River” prairie cordgrass (Spartina pectinata Link) and “Cave-in-Rock” switchgrass (Panicum virgatum L.) was determined by evaluating seed germination, plant growth, ion uptake, and leaf anatomical feature responses in saline conditions. Red River seeds retained 50% of germination potential under high salinity up to 300 mM NaCl, whereas Cave-in-Rock seed germination was reduced by 80% at 300 mM NaCl. Red River seedlings survived up to 500 mM NaCl, while more than 30% of Cave-in-Rock did not survive above 100 mM NaCl in greenhouse experiments. Red River produced more biomass and second-generation tillers and had a greater root and shoot biomass ratio than Cave-in-Rock under all salinity ranges. Sodium accumulation in shoots of Cave-in-Rock increased with increasing salinity, whereas Red River maintained a low level of sodium in biomass through salt-gland exclusion. The increasing rate of selectivity coefficient for potassium over sodium in Red River was higher than in Cave-in-Rock with increasing salinity. Although seed germination and plant growth decreased as salinity increased, Red River retained its potential of seed germination and to produce new tillers and biomass under salinity stress.