  We report the discovery of a one magnitude increase in the optical brightness
of the 59.63 minute orbital period AM CVn binary SDSS J113732.32+405458.3.
Public $g$, $r$, and $i$ band data from the Zwicky Transient Facility (ZTF)
exhibit a decline over a 300 day period, while a few data points from
commissioning show that the peak was likely seen. Such an outburst is likely
due to a change in the state of the accretion disk, making this the longest
period AM CVn binary to reveal an unstable accretion disk. The object is now
back to its previously observed (by SDSS and PS-1) quiescent brightness that is
likely set by the accreting white dwarf. Prior observations of this object also
imply that the recurrence times for such outbursts are likely more than 12
years.
