We report the discovery of gamma-ray pulsations from the nearby isolated millisecond pulsar PSR J0030+0451 with the Large Area Telescope (LAT) on the \emph{Fermi} Gamma-ray Space Telescope (formerly GLAST). This discovery makes PSR J0030+0451 the second millisecond pulsar to be detected in gamma-rays after PSR J0218+4232, observed by the EGRET instrument on the Compton Gamma Ray Observatory. The spin-down power $\dot E = $ 3.5 $\times$ 10$^{33}$ ergs s$^{-1}$ is an order of magnitude lower than the empirical lower bound of previously known gamma-ray pulsars. The emission profile is characterized by two narrow peaks, respectively 0.07 $\pm$ 0.01 and 0.08 $\pm$ 0.02 wide, separated by 0.44 $\pm$ 0.02 in phase. The first gamma-ray peak falls 0.15 $\pm$ 0.01 after the main radio peak. The pulse shape is similar to that of the "normal" gamma-ray pulsars. An exponentially cut-off power-law fit of the emission spectrum leads to an integral photon flux above 100 MeV of (6.76 $\pm$ 1.05 $\pm$ 1.35) $\times 10^{-8}$ cm$^{-2}$ s$^{-1}$ with cut-off energy (1.7 $\pm$ 0.4 $\pm$ 0.5) GeV. Based on its parallax distance of $(300 \pm 90)$ pc, we obtain a gamma-ray efficiency $L_\gamma / \dot{E} \simeq 15%$ for the conversion of spin-down energy rate into gamma-ray radiation, assuming isotropic emission.Read less <