Our study determined the impacts of silicon (Si) supplementation on Medicago truncatula inoculated with Ensifer meliloti rhizobial strains that differed in their capacity for nitrogen fixation: Sm1021 (‘low-efficiency’) or Sm1022 (‘high-efficiency’). We then examined how Si and rhizobial efficacy influence plant resistance to a polyphagous insect, Helicoverpa armigera. These combinations were supplied with Si or untreated in a glasshouse experiment, where we quantified nodule flavonoids and foliar chemistry (free amino acids, soluble protein, elemental C, N and Si). Si supply increased nodule number per plant, specific nodule flavonoids, contents of foliar nitrogenous compounds and foliar C, but not foliar Si. We also demonstrated that rhizobial efficacy altered the magnitude of Si effects on various traits. For example, Si significantly promoted concentrations of foliar N and soluble protein in the plants associated with the ‘low-efficiency’ strain only and this was not the case with the ‘high-efficiency’ one. Additionally, increases in foliar free amino acids in response to Si addition did not increase susceptibility to H. armigera. Collectively, our study indicates that Si enrichment generates positive effects on M. truncatula, particularly when the association with rhizobia is relatively inefficient, and may play a more prominent role in rhizobial functionality than previously thought.