We have used double-resonance excitation of a laser-vaporized sample of atomic boron to measure the energies of np Rydberg states from n∼35 to 70, to a precision of ±0.02 cm −1 . The 2s 23s state was excited resonantly using light at 249 nm from a frequency-doubled narrowband pulsed dye laser; subsequently, 2s 2 np Rydberg states were excited using a second frequency-doubled dye laser operating between 375 and 372 nm . Energy calibration of the spectrum was done by comparison to the laser-induced fluorescence spectrum of molecular iodine. The Rydberg spectrum was fit to a simple Rydberg formula with excellent agreement, yielding a quantum defect of μ=0.527±0.002 and an ionization energy relative to the 2s 23s intermediate state of I(2 s 23 s)=26,888.38±0.02 cm −1 . Taken together with the previously determined best value for the energy of the 2s 23s state, we determine the ionization energy of the ground state to be 66,928.06±0.03 cm −1 , in agreement with the best previous results, but with an uncertainty which is more than a factor of three smaller.