Models of visual salience detection rely on center-surround interactions, yet it remains unclear how these computations are distributed across retinal, cortical, and subcortical circuits due to their overlapping contributions. Here, we reveal a de novo collicular mechanism for surround suppression by combining patterned optogenetics with whole-cell recordings from individual neurons in the mouse superficial superior colliculus (SCs). Center zones were defined by monosynaptic input from channelrhodopsin-expressing retinal ganglion cells in collicular midbrain slices. Surround network optoactivation suppressed center responses compared to center-only input. This suppression is excitatory in origin, arising from the withdrawal of center excitation via surround-driven inhibition of local recurrent excitatory circuits, as demonstrated by cell-type-specific trans-synaptic tracing and computational modeling. These findings identify a local circuit mechanism for saliency computation in the SCs, independent of cortical input.