Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well-established as heterogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large-pore COF as catalytic support in α,ω-diene ring-closing metathesis reactions, leading to increased macrocylization selectivity. COF pore-wall modification by immobilization of a Grubbs-Hoveyda-type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF-catalyst (MMC:O = 1.35) of up to 51% compared to the homogeneous catalyst (MMC:O = 0.90) was observed along with a substrate-size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement. This dataset contains all data from analytical measurements including FT-IR spectra, raw XRD patterns, 1H and 13C ssNMR and lNMR spectra, TGA, N2 sorption isotherms, pore-size distributions, BET plots, SEm and TEM images, quantum-chemically optimized structures, CIFs of the journal article mentioned under the related publication. Open .cif and .xyz files with a visualization software (see http://ww1.iucr.org/iucr-top/cif/), .raw files with WinXPOW, .sp files with PerkinElmer Spectrum software, .mnova files with MestReNova, .fq/.gr/iq/sq files with a text editor, .qps with ASiQwin, and *.pro files with TOPAS.