Atypical asymmetries of spatial attention have been reported in children with attention deficit hyperactivity disorder (ADHD) and may be exacerbated by non-spatial factors such as attentional capacity. Although preliminary evidence suggests that asymmetries of attention in ADHD may be modifiable by the psychostimulant, methylphenidate, further placebo-controlled studies are required. This study first aimed to confirm recent evidence that increasing non-spatial processing load at fixation can unmask a spatial gradient of target detection in children with ADHD but not Controls. Second, we used placebo-controlled randomised trial methodology to ask whether 20 mg of methylphenidate (MPH) could remediate any load-dependent asymmetry of spatial attention in adolescents with ADHD. Twelve male adolescents with ADHD were assessed twice in a double-blind, randomized design, under either placebo or an acute dose of methylphenidate. Thirteen typically developing adolescent Controls completed a single session under placebo. Participants completed a computer-based task in which they monitored a centrally presented rapid serial visual presentation stream for a probe stimulus, while also responding to brief peripheral events. The attentional load of the central task was manipulated by varying the target instructions but not the physical stimuli or the frequency of targets. Between-group analyses under placebo conditions indicated that increased attentional load induced a spatial gradient for target detection in the ADHD but not Controls, such that load slowed response times for left, but not, right hemi-field targets. This load-dependent spatial asymmetry in the adolescents with ADHD was abolished by administration of methylphenidate. Methylphenidate may “normalize” target detection between the hemi-fields in ADHD via enhancement of the right-lateralised ventral attention networks that support non-spatial attention.