  The first determination of the nuclear charge radius by laser spectroscopy
for a five-electron system is reported. This is achieved by combining
high-accuracy ab initio mass-shift calculations and a high-resolution
measurement of the isotope shift in the $2s^2 2p\, ^2\mathrm{P}_{1/2}
\rightarrow 2s^2 3s\, ^2\mathrm{S}_{1/2}$ ground state transition in boron
atoms. Accuracy is increased by orders of magnitude for the stable isotopes
$^{10,11}$B and the results are used to extract their difference in the
mean-square charge radius $\langle r^2_\mathrm{c}\rangle^{11} - \langle
r^2_\mathrm{c}\rangle^{10} = -0.49\,(12)\,\mathrm{fm}^2$. The result is
qualitatively explained by a possible cluster structure of the boron nuclei and
quantitatively used as a benchmark for new ab initio nuclear structure
calculations using the no-core shell model and Green's function Monte Carlo
approaches.
