Gold nanoparticles dispersed in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid (Au-BMI·PF(6)) were supported in chitin (CTN) chemically crosslinked with glyoxal and epichlorohydrin to obtain a new supported ionic liquid phase (SILP) catalyst with high catalytic activity, and providing an excellent environment for enzyme immobilization. This modified biopolymer matrix (Au-BMI·PF(6)-CTN) was used as a support for the immobilization of the enzyme peroxidase (PER) from pea (Pisum sativum), and employed to develop a new biosensor for rosmarinic acid (RA) determination in pharmaceutical samples by square-wave voltammetry. In the presence of hydrogen peroxide, the PER catalyzes the oxidation of RA to the corresponding o-quinone, which is electrochemically reduced at a potential of +0.14 V vs. Ag/AgCl. Under optimized conditions, the resulting peak current increased linearly for the RA concentration range of 0.50 to 23.70 μM with a detection limit of 70.09 nM. The biosensor demonstrated high sensitivity, good repeatability and reproducibility, and long-term stability (15% decrease in response over 120 days). The method was successfully applied to the determination of RA content in pharmaceutical samples, with recovery values being in the range of 98.3 to 106.2%. The efficient analytical performance of the proposed biosensor can be attributed to the effective immobilization of the PER enzyme in the modified CTN matrix, the significant contribution of the high conductivity of the ionic liquid, the facilitation of electron transfer promoted by gold nanoparticles, and the inherent catalytic ability of these materials.