The allocation of attention to a given moment in time has been shown to confer benefits for perceiving and rapidly responding to isolated stimuli. Here we examined whether temporal preparation (TP) also improves the spatial selection of a stimulus and its associated response among competing alternatives. In a series of 3 experiments, we combined a flanker compatibility task, in which a central target letter was flanked by various (compatible, neutral or incompatible) lateral distractor letters to manipulate selection difficulty, with the constant-foreperiod paradigm, to manipulate TP. We found a reduced flanker compatibility effect with high (vs. low) degrees of TP. This boost of space-based target selection argues for synergies between TP and spatial attention. However, when response conflict was enhanced by having the flanking distractors precede target onset by 32 ms, the effect of TP was reversed, suggesting that a high degree of TP can also increase response selection difficulty, possibly by enhancing general response readiness. Taken together, TP appears to modulate speeded spatial selection in conflict situations, producing net performance benefits or costs depending on the relative strength of stimulus- vs. response-related types of conflict.