The new compound H 3 P⋯AgI has been synthesized in the gas phase by means of the reaction of laser-ablated silver metal with a pulse of gas consisting of a dilute mixture of ICF 3 and PH 3 in argon. Ground-state rotational spectra were detected and assigned for the two isotopologues H 3 P⋯ 107 AgI and H 3 P⋯ 109 AgI in their natural abundance by means of a chirped-pulse, Fourier-transform, microwave spectrometer. Both isotopologues exhibit rotational spectra of the symmetric-top type, analysis of which led to accurate values of the rotational constant B 0 , the quartic centrifugal distortion constants D J and D JK , and the iodine nuclear quadrupole coupling constant χ aa (I) = eQq aa . Ab initio calculations at the explicitly-correlated level of theory CCSD(T)(F12∗)/aug-cc-pVDZ confirmed that the atoms P⋯Ag-I lie on the C 3 axis in that order. The experimental rotational constants were interpreted to give the bond lengths r 0 (P⋯Ag) = 2.3488(20) Å and r 0 (Ag-I) = 2.5483(1) Å, in good agreement with the equilibrium lengths of 2.3387 Å and 2.5537 Å, respectively, obtained in the ab initio calculations. Measures of the strength of the interaction of PH 3 and AgI (the dissociation energy D e for the process H 3 P⋯AgI = H 3 P + AgI and the intermolecular stretching force constant F P⋯Ag ) are presented and are interpreted to show that the order of binding strength is H 3 P⋯HI