This study aimed to evaluate the yield and chemical composition of essential oil (EO) and the growth parameters of Melissa officinalis L. (lemon balm) inoculated with arbuscular mycorrhizal fungi (AMF) under different rates of phosphorus (P). Treatments comprised a high and low P rate combined or not with inoculation of Rhizophagus clarus (C. Walker & A. Schüßler) or Claroideoglomus etunicatum (C. Walker & A. Schüßler), arranged according to a 2 × 3 factorial design with 8 replications. At 4 months after transplanting, shoot fresh and dry weights increased in both AMF-inoculated treatments under a low P rate. There was an increase in shoot P content with C. etunicatum inoculation under a high P rate. EO yields ranged from 0.04% to 0.12% and increased with R. clarus inoculation. Eighteen compounds were identified in EOs. The major components were geranial (43.96%–54.93%), neral (29.95%–34.66%), geraniol (3.11%–12.85%), and (E)-caryophyllene (2.62%–6.66%). It was concluded that AMF inoculation increased plant growth, improved EO yield, and modified EO composition. It is recommended to inoculate lemon balm with R. clarus under low P rates.