  We report the discovery of a brightness enhancement in the center of a large
sunspot umbra at a wavelength of 3 mm using the Atacama Large
Millimeter/sub-millimeter Array (ALMA). Sunspots are amongst the most prominent
features on the solar surface, but many of their aspects are surprisingly
poorly understood. We analyzed a {\lambda}=3 mm (100 GHz) mosaic image obtained
by ALMA, which includes a large sunspot within the active region AR12470 on
December 16, 2015. The 3 mm map has a field-of-view and spatial resolution,
which is the highest spatial-resolution map of an entire sunspot in this
frequency range. We find a gradient of 3 mm brightness from a high value in the
outer penumbra to a low value in the inner penumbra/outer umbra. Within the
inner umbra, there is a marked increase in 3mm brightness temperature, which we
call an umbral brightness enhancement. This enhanced emission corresponds to a
temperature excess of 800 K relative to the surrounding inner penumbral region
and coincides with excess brightness in the 1330 and 1400 {\AA} slitjaw images
of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial
lightbridge. This {\lambda}=3 mm brightness enhancement may be an intrinsic
feature of the sunspot umbra at chromospheric heights, such as a manifestation
of umbral flashes, or it could be related to a coronal plume since the
brightness enhancement was coincident with the footpoint of a coronal loop
observed at 171 {\AA}.
