Poly(styrenesulfonic acid)-functionalized materials based on poly(styrenesulfonic acid sodium salt) incorporated via aqueous atom transfer radical polymerization (ATRP) initiated from the surface of large-pore mesoporous SBA-15 silica support have been synthesized. The inorganic–organic nature of these hybrid materials makes them particularly desirable for acid-catalyzed reactions that require extended and hydrophobic surface areas with a narrow pore diameter distribution in the mesoporous range. Acidic hybrid materials were prepared by grafting the ATRP-initiator (3-(chlorodimethylsilyl)propyl bromoisobutyrate) on the silica surface, subsequent polymerization of the styrenesulfonic acid sodium salt monomer, and final sodium ion exchange by acid activation. Conventional and ultra-large-pore SBA-15 silica supports with nominal (BJH) pore diameter ranging from 8 to 32 nm were used for the incorporation of different polymer loadings at different polymerization times. The silylation of ATRP-initiator-functionalized SBA-15 supports has allowed a better control of the ATRP within the mesoporous structure. The use of ultra-large-pore SBA-15 supports provides a remarkable increase of the porosity which allowed us to properly allocate the polymer. The hybrid poly(styrenesulfonic acid)-modified materials showed good catalytic activities in the esterification of oleic acid with n-butanol, particularly in terms of intrinsic activity per acid site.