The 1-(trimethylsilyl)bicyclobutonium ion is generated from (1’-(trimethylsilyl)cyclopropyl)methanol by reaction with SbF5. The NMR spectroscopic data in SO2ClF/SO2F2 solution at -128 degrees C are in accord with a bridged puckered bicyclobutonium structure undergoing a 3-fold rapid degenerate rearrangement that renders the two beta- and one gamma-methylene groups equivalent, leading to one averaged C-13-NMR signal for the CH2 groups at 48.9 ppm. Conformational ring inversion is slow so that two separate signals for the three averaged endo-CH2 (4.04 ppm) and three averaged exo-CH2 hydrogens (3.24 ppm) are observed. The deuterium equilibrium isotope effects for exe- and endo-CHD-labeled cations are different in sign and magnitude and are rationalized by different endo- and exo-C-H bond force constants at the pentacoordinated carbon. NMR chemical shift calculations for the 1-silylbicyclobutonium ion and the (1’-silylcyclopropyl)methyl cation were performed with the GIAO-SCF and GIAO-MP2 methods. The experimental shifts are satisfactorily reproduced by GIAO-MP2/tzp/dz calculated shifts for the 1-silylbicyclobutonium structure. The good agreement between theory and experiment supports a fully degenerate set of interconverting 1-(trimethylsilyl)bicyclobutonium ions and excludes contributions from other isomers to the observed equilibrium process. The geometric and electronic properties of the 1-(trimethylsilyl)bicyclobutonium cation are intermediate between those of the parent bicyclobutonium ion and those of the methyl-substituted analogue.