<p>The addition of agri-food waste to the biodegradable polymer matrix supports the circular economy. In this paper, lignocellulosic residues of beetroot pulp (BP) are modified using (3-aminopropyl)triethoxysilane (APTES). Moreover, the influence of an epoxy-based chain extender (Joncryl^® ADR 4468 (ADR)) on the properties of the poly(lactic acid) (PLA) biocomposites was investigated. BP as a PLA filler, as well as the co-equal roles of silanisation and ADR addiction, have not been widely tested. The chemical composition of the BP filler was determined. The success of chemical modification was confirmed by FTIR and SEM-EDS analyses. DSC, DMA and TGA were applied for the characterisation of thermal properties of the manufactured biocomposites. The influence of APTES modification and ADR on the biocomposite on the molecular weight of PLA, water resistance, rheological and mechanical properties was tested. Results showed that adhesion between filler and matrix is enhanced after APTES modification according to SEM observation. APTES modification of the BP and ADR addition has a slight influence on reducing water absorption; however, the combination of both methods significantly increases water resistance. It can be justified by the possible reaction between the amino group of APTES and the epoxy group of ADR. Rheological and molecular weight studies showed that ADR counteracts the effects of PLA hydrolysis at elevated temperatures thanks to the PLA branching process. The tensile strength increased after the silane modification by 28% compared to unmodified BP composites, which confirms enhanced adhesion between components. Tensile strength increases despite a decrease in molecular weight, which confirms the effect of increased adhesion at the interface, counteracting the effect of the aminolysis reaction. Results are promising to produce ecological biocomposites with satisfying mechanical, thermal and rheological properties from agri-food residue.</p>