  We study the properties of the $\Theta^+$ in nuclear matter and $\Theta^+$
hypernuclei within the quark mean-field (QMF) model, which has been
successfully used for the description of ordinary nuclei and $\Lambda$
hypernuclei. With the assumption that the non-strange mesons couple only to the
$u$ and $d$ quarks inside baryons, a sizable attractive potential of the
$\Theta^+$ in nuclear matter is achieved as a consequence of the cancellation
between the attractive scalar potential and the repulsive vector potential. We
investigate the $\Theta^+$ single-particle energies in light, medium, and heavy
nuclei. More bound states are obtained in $\Theta^+$ hypernuclei in comparison
with those in $\Lambda$ hypernuclei.
