Nanocomposites of metal nanoparticles (NPs) and bacterial nanocellulose (BC) enable fabrication of softand biocompatible materials for optical, catalytic, electronic and biomedical applications. Current BC-NP nanocomposites are typically preparedbyin situ synthesis of the NPs or electrostatic adsorption of surface functionalized NPs, whichlimitspossibilities to control and tune NP size, shape, concentration and surface chemistryand influence the properties and performance of the materials. Here we describe a self-assembly strategy for fabrication of complex and well-defined BC–NP composites using colloidal gold and silver NPs ofdifferent sizes, shapes and concentrations. The self-assembly processresults innanocompositeswith distinct biophysical and optical properties.In addition to antibacterial materials and materials with excellent senor performance, materials with unique mechanoplasmonic properties are developed. The homogenous incorporation of plasmonic gold NPs in the BC enablesextensivemodulation of the optical properties by mechanical stimuli. Compressiongives rise to near-field coupling between adsorbed NPs, resultingin tunable spectral variations and enhanced broadband absorptionthat amplifyboth non-linear optical and thermoplasmonic effectsand enablesnovel biosensing strategies.