Despite the well-demonstrated modulatory effect of serotonin on spinal motor circuits, the specific roles and identity of the involved serotonergic neurons remain unclear. This ambiguity owes to the anatomical and neurochemical heterogeneity of the medullary brainstem regions housing the spinally-projecting serotonergic neurons, and to the limited number of studies investigating their connectivity and function cell-specifically.Here, we selectively target the midline serotonergic neurons of the caudal medulla (mCM-5HT) in the mouse to explore their output connectivity in the spinal cord and their influence on spinal circuits and motor behaviors. Using anterograde viral tools, we show uninterrupted serotonergic projections from mCM-5HT neurons all along the spinal cord, including the lumbar ventral horn where core circuits for locomotion reside. Then, by optogenetic activation in ex vivo brainstem-spinal cord preparations (presented previously, Giorgi et al 2021), we find that mCM-5HT neurons do not recruit lumbar motoneurons. Instead, they consistently inhibit motor responses to sensory inputs, suggesting a role in gating of sensory signals to motoneurons.To test if sensory processing by mCM-5HT neurons could affect motor output, we optogenetically manipulated mCM-5HT neurons in vivo during both standard (open field) and skilled (horizontal ladder) locomotion, when reliance on sensory processing is likely critical. In the open field test, activation or deactivation of mCM-5HT neurons neither initiate nor halt locomotion, nor does it affect its speed. In contrast, their deactivation on the horizontal ladder disrupts paw placement accuracy. Altogether, these results underscore the role of mCM-5HT neurons in sensory information processing necessary for adaptive movement.