During visual detection with saccades, a target with higher luminance is detected with reduced reaction times. In such visual detection behaviors, luminance-related sensory signals should be converted into movement-related signals for saccade initiation. At the site where the visuomotor transformation takes place, there is the possibility that visual activity not only encodes the target luminance but also affects the generation of an upcoming saccade. To assess this possibility, we recorded single-cell activity from visually responsive neurons in the lateral intraparietal area (LIP) when monkeys made a saccade to an isolated target over five luminance levels. We found that as stimulus luminance increased, visual response strength increased, and response onset latency decreased. These luminance-related changes in activity were significantly correlated with changes in reaction time. In particular, changes in response onset latency accounted for a substantial part of the observed changes in reaction time, suggesting that luminance-related changes in response onset latency may propagate to the saccade generation process. However, the length of time from response onset to saccade onset was not constant but increased as luminance was reduced, suggesting the existence of other luminance-dependent processing in downstream and/or parallel pathways before saccade generation. Additionally, we failed to find strong covariance between response strength or latency and reaction time when the effect of luminance changes was removed. Thus, the present results reveal how visually responsive LIP neurons contribute to saccade generation in visual detection.