A 70 ml dose of dietary nitrate (NO₃ ^-) has shown to improve exercise performance over a variety of distances. However, research has typically demonstrated NO₃ ^- efficacy within a tightly controlled laboratory environment. Therefore, this study aimed to understand if NO₃ ^- could improve the time to complete an outdoor 5 km time-trial. In a double-blind, placebo-controlled, randomised cross-over design, 103 physically active runners (Mean ± SD; age = 41 ± 11y; personal best = 25:45 ± 5:11 min:s; number of previous parkruns = 24 ± 21) volunteered to participate. Four outdoor 5 km running time-trials were performed (two baseline and two experimental), each separated by one week. Experimental trials consisted of participants consuming 70 ml of NO₃ ^- (∼4 mg) and a nitrate-depleted placebo two hours prior experimentation. Baseline was performed before (pre-baseline) and after (post-baseline) experimental conditions. Compared to pre-baseline, time to complete 5 km improved with the ingestion of NO₃ ^- (–30 ± 50s, 95% CI =  –17 to –43s, P < 0.001) and placebo (–30 ± 49s, 95% CI = –17 to –43s, P < 0.001). No differences were shown between pre-baseline and post-baseline (–16 ± 80s, 95% CI = –5 to 37s,  .128) and NO₃ ^- and placebo (–0 ± 52s, 95% CI = –14 to 13, .974). An acute dose of NO₃ ^- improved time to complete an outdoor 5 km time-trial, but this improvement may be attributable to placebo effects. The expectation of receiving NO₃ ^- appeared to have had a substantial improvement on performance regardless of the presence of NO₃ ^–.