A measurement of jet substructure observables is presented using tt events in the lepton+jets channel from proton-proton collisions at √s = 13 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb−1. Multiple jet substructure observables are measured for jets identified as bottom, light-quark, and gluon jets, as well as for inclusive jets (no flavor information). The results are unfolded to the particle level and compared to next-to-leading-order predictions from POWHEG interfaced with the parton shower generators PYTHIA 8 and HERWIG 7, as well as from SHERPA 2 and DIRE 2. A value of the strong coupling at the Z boson mass, αS(mZ) = 0.115+−0.0150.013, is extracted from the substructure data at leading-order plus leading-log accuracy.