Uniformly dispersed ultrafine platinum nanoparticles confined in a covalent organic framework (Pt/COF) have been designed and synthesized, which exhibit good catalytic activities in both enzyme-like and electrocatalytic catalysis. Benefiting from the space-confinement effect of pores in the COF matrix, the size of in situ grown Pt nanoparticles is as small as 2.44 nm with a narrow size distribution. Owing to the structure superiority, the Pt/COF catalyst exhibits much better peroxidase/oxidase-like activity than unsupported Pt nanoparticles and a physical mixture of the two components. Based on the inhibition of catalytic oxidation of the peroxidase substrate by Pt/COF, a sensitive colorimetric method is established for tannic acid sensing. Furthermore, the Pt/COF catalyst also exhibits better electrocatalytic activity and stability than commercial Pt/C catalyst towards the methanol oxidation reaction (MOR). This work demonstrates the promising application potential of COF-supported materials in both enzyme-mimetic and electrocatalytic catalysis.