  We report the discovery of TeV gamma-ray emission from the Type Ia supernova
remnant (SNR) G120.1+1.4, known as Tycho's supernova remnant. Observations
performed in the period 2008-2010 with the VERITAS ground-based gamma-ray
observatory reveal weak emission coming from the direction of the remnant,
compatible with a point source located at $00^{\rm h} \ 25^{\rm m} \ 27.0^{\rm
s},\ +64^{\circ} \ 10^{\prime} \ 50^{\prime\prime}$ (J2000). The TeV photon
spectrum measured by VERITAS can be described with a power-law $dN/dE =
C(E/3.42\;\textrm{TeV})^{-\Gamma}$ with $\Gamma = 1.95 \pm 0.51_{stat} \pm
0.30_{sys}$ and $C = (1.55 \pm 0.43_{stat} \pm 0.47_{sys}) \times 10^{-14}$
cm$^{-2}$s$^{-1}$TeV$^{-1}$. The integral flux above 1 TeV corresponds to $\sim
0.9%$ percent of the steady Crab Nebula emission above the same energy, making
it one of the weakest sources yet detected in TeV gamma rays. We present both
leptonic and hadronic models which can describe the data. The lowest magnetic
field allowed in these models is $\sim 80 \mu$G, which may be interpreted as
evidence for magnetic field amplification.
