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Silk biomaterials are widely used in tissue engineering applications due to their mechanical tuneability, processability and low immunogenic response. However, due to the lack of cell binding sites, further modification is typically required to optimize silk biomaterials towards a specific biomedical application. Plasma immersion ion implantation (PIII) is a surface modification technique that can covalently attach a wide range of bioactive molecules onto silk surfaces to trigger desirable biological responses. However, the effect of PIII treatment on the surface and bulk properties of silk biomaterials remains largely unknown. Therefore, the aim of this study is to investigate PIII-treated silk (PIII-silk) biomaterials towards their use as a platform for biomedical applications. Silk films with or without PIII treatment were examined to determine changes to the surface and bulk properties, including the chemical, mechanical, degradation properties, and the in vivo immune response. PIII treatment of silk was found to alter surface chemistry with densification of the silk surface layer and the introduction of oxygen and nitrogen functional groups. This then resulted in a significant increase in surface stiffness but also led to minor changes in tensile properties of clinically relevant silk formats and minor differences in in vitro degradation properties. Following PIII treatment, there was a significant increase in HCAEC adhesion on PIII-silk compared to untreated silk but no significant differences in fibrous encapsulation and macrophage response when implanted in vivo in a subcutaneous mouse model. Together, these results show that PIII treatment of silk minimally compromises the bulk properties and can be used to promote cellular interactions as well as a means of reagent-free, covalent immobilization of biomolecules.