We describe an experiment which demonstrates the feasibility of trapping significant quantities of short-lived radioactive atoms with laser light. A thermal beam of 22.5 sec half-life $^{21}\mathrm{Na}$ atoms was produced on-line at the LBL 88'' Cyclotron. After decelerating the beam using a Zeeman-tuned slowing technique we stored about 4\ifmmode\times\else\texttimes\fi{}${10}^{3}$ $^{21}\mathrm{Na}$ atoms in a magneto-optical trap. The number of trapped atoms is large enough to be used in experimental studies of the beta decay of $^{21}\mathrm{Na}$. The basic method can be adapted for other rare isotopes.