Arbuscular mycorrhizal fungi (AMF) species and/or phosphorus addition in the soil can affect secondary metabolism production and growth in plants. This study evaluated these effects on Piper divaricatum culture, which presents an essential oil rich in phenylpropanoids. Growth parameters, leaf volatile composition, total phenolic content, and the enzymatic activity of lipoxygenase (LOX) and phenylalanine ammonia-lyase (PAL) were monitored. At 90 days post inoculation (dpi), the treatments with AMF and AMF + P were more effective in terms of the number and biomass of leaves. In addition, the AMF group had increased plant height and root length. Phenylpropanoid and methyl eugenol contents were higher at 30 dpi in the P and AMF + P association treatments. However, at 90 dpi, the plants with P addition at 20 mg·dm−3 and AMF + 200 mg of P dm−3 showed higher phenolic contents and PAL activity, respectively. All treatments increased LOX activity, especially with P addition at 200 mg dm−3. These results demonstrate that P and AMF can be applied to optimize leaf biomass and volatile compound production in P. divaricatum.