Using the CHARA Array and the Palomar Testbed Interferometer, the chemically peculiar star $lambda$ Bo{o}tis has been spatially resolved. We have measured the limb darkened angular diameter to be $theta_{LD} = 0.533pm0.029$ mas, corresponding to a linear radius of $R_{star} = 1.70 pm 0.10 R_odot$. The measured angular diameter yields an effective temperature for $lambda$ Boo of $T_{eff} = 8887 pm 242$ K. Based upon literature surface gravity estimates spanning $log{(g)} = 4.0-4.2$ $[rm{cm s}^{-rm{2}}]$, we have derived a stellar mass range of $M_{star} = 1.1 - 1.7$ $M_odot$. For a given surface gravity, the linear radius uncertainty contributes approximately $sigma(M_star) = 0.1-0.2 M_odot$ to the total mass uncertainty. The uncertainty in the mass (i.e., the range of derived masses) is primarily a result of the uncertainty in the surface gravity. The upper bound of our derived mass range ($log(g)=4.2, M_star = 1.7pm0.2 M_odot$) is consistent with 100-300 MYr solar-metallicity evolutionary models. The mid-range of our derived masses ($log(g)=4.1, M_star = 1.3pm0.2 M_odot$) is consistent with 2-3 GYr metal-poor evolutionary models. A more definitive surface gravity determination is required to determine a more precise mass for $lambda$ Boo.
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