A new system coupling sub/supercritical water extraction and hydrolysis with in-line solid-phase extraction (SPE) and on-line high-performance liquid chromatography (HPLC) was assembled and validated. The new system was conceptually divided into two modules, one based on extraction, hydrolysis, and SPE, and the other dedicated to the HPLC. The modules can be operated independently or simultaneously. This work aims to present the system validation using black grape peel as raw material, a by-product rich in bioactive compounds and lignocellulose, an attractive substrate for extraction and hydrolysis. The extraction and hydrolysis processes were carried out in a semi-continuous mode. The solid raw material (10 g) was loaded in an extraction cell, through which water was continuously flowed (2 mL/min). Extracts or hydrolysates leaving the cell flowed to the supercritical reactor, which residence time was approximately 1 second, and further to the SPE column. The SPE column was packed with adsorbent (PoraPak™ Rxn), activated with 30 mL ethanol, and conditioned with 30 mL water. The extraction conditions were: 45 °C (extraction cell) and 15 MPa. The hydrolysis conditions were: 150 °C (extraction cell), 250 °C (supercritical reactor), and 20 MPa. During extraction and hydrolysis, the system automatically injected a sample fraction (5 µL) in the HPLC module, from which the concentrations of anthocyanins and sugars were recovered. The real-time monitoring of the products enabled quick and efficient responses, less process time, low solvent consumption, fewer analytical errors, and less labor. The obtention of bioactive compounds can increase the industry’s profitability since they have a high market value and are highly requested in the food, pharmaceutical, and cosmetic sectors. In addition, the lignocellulosic hydrolysate, rich in fermentable sugars, can be applied to produce biofuels. Therefore, this system can be used in the management of agro-industrial by-products in a biorefinery concept.