Sustainable production of biomass crops is important in the development of feedstocks for the production of biofuels and other bioproducts. This study investigates the use of nine beneficial soil microbes and a plant biostimulant (i.e., Ascophyllum nodosum seaweed extract) to increase the growth of two giant Miscanthus (Miscanthus × giganteus) cultivars, ‘Amuri’ and ‘Nagara’, under greenhouse conditions and in the field on poor-quality, marginal land. Greenhouse trials indicated increases in shoot dry weight (DW) in ‘Amuri’ in treatments with Gluconacetobacter diazotrophicus PAL5T LsdB⁺⁺, Gluconacetobacter johannae UAP-Cf-76, and Variovorax paradoxus JM67 by 15%–24% compared with untreated controls. In ‘Nagara’, shoot DW was increased in the treatments with Penicillium bilaiae by 11% and the seaweed extract by 10%. The nutrient content of shoot tissues increased in the same treatments in which biomass was increased. Despite a lack of treatment effects on shoot DW in ‘Amuri’ in the field, several treatments increased Fe and Zn content in shoots by up to 1.9×. In ‘Nagara’ in the field, treatment with G. johannae UAP-Cf-76 and the seaweed extract resulted in increases in shoot DW by 16% and 23%, respectively, and several treatments resulted in increases in shoot Fe and Zn concentrations. The productivity enhancements in giant Miscanthus by beneficial soil microbes and the seaweed extract may be associated with increasing access to limited soil nutrients. These findings suggest that the use of beneficial soil microbes and plant biostimulants may aid in the sustainable production of giant Miscanthus on marginal lands.