  We present a new upper limit on CMB circular polarization from the 2015
flight of SPIDER, a balloon-borne telescope designed to search for $B$-mode
linear polarization from cosmic inflation. Although the level of circular
polarization in the CMB is predicted to be very small, experimental limits
provide a valuable test of the underlying models. By exploiting the non-zero
circular-to-linear polarization coupling of the HWP polarization modulators,
data from SPIDER's 2015 Antarctic flight provide a constraint on Stokes $V$ at
95 and 150 GHz from $33<\ell<307$. No other limits exist over this full range
of angular scales, and SPIDER improves upon the previous limit by several
orders of magnitude, providing 95% C.L. constraints on $\ell
(\ell+1)C_{\ell}^{VV}/(2\pi)$ ranging from 141 $\mu K ^2$ to 255 $\mu K ^2$ at
150 GHz for a thermal CMB spectrum. As linear CMB polarization experiments
become increasingly sensitive, the techniques described in this paper can be
applied to obtain even stronger constraints on circular polarization.
