Although phosphate ion (hereafter “P”) sorption is known to influence clay surface reactivity and trace metal behavior in the environment, there is still a lack of in situ spectroscopic evidence on the identity of phosphate sorption species formed at the interface between solution and clays with a 2:1 layered structure (2 silica tetrahedral: 1 octahedral layers), which are ubiquitous in soils and clay formations. We studied the surface speciation of phosphate ions by in situ spectroscopic monitoring along P sorption at the interface between a solution and a homoionic Na-illite, as a function of time or aqueous phosphate concentration (20–250 µM) leading to low/moderate P surface coverage. We also combined them with batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated that the percentage of P sorption depends on pH, aqueous phosphate concentration, and the clay/solution ratio of the experiment, i.e., the coverage of the clay surface by P, with a maximal sorption capacity of the clay equal to ca. 6 µmol.g−1 at pH 4. Macroscopic and EM data further suggest that P sorption mechanisms onto illite confer negative charges on the surface and involve several sorption species and/or surface sites present on the clay edges. In situ FTIR spectra showed that, at acidic pH, three types of phosphate surface species exist at the illite-solution interface: an outer-sphere surface complex (OSSC) of phosphate (ΞSOH2+…H2PO4-, with S referring to a sorption site on the clay), an inner-sphere surface complex (ISSC) of phosphate, probably a monodentate binuclear surface complex ((ΞSO)2PO2) formed by surface ligand exchange with high-affinity aluminol sites and, to a lesser extent, low-affinity sites on the clay edges, and, in limited amounts, an Al-phosphate surface precipitate. Phosphate ion sorption occurs through the initial formation of the OSSC of phosphate, which dominates phosphate surface speciation at short reaction times and low P concentrations and transforms over time into the ISSC of phosphate. This study presents the first published in situ surface speciation of phosphate ions sorbed at the interface between a solution and an illite. The evidence of a strong sorption of phosphate ions at this clay surface provides new and sound knowledge to better understand the environmental fate of phosphate ions and trace metals, particularly in agricultural soil - groundwater continua or in clay host rocks considered for high-level radioactive waste disposal.