Nitric oxide is a key regulator of many systems in the human body and has become a popular supplement for sports, given its potential to increase health and physical performance. Spinach extract contains a rich amount of nitrates, and we aim to examine the effects of its supplementation on physical performance, body composition, and safety profile. A systematic review following the guidelines from the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) was performed in PubMed, Embase, and Cochrane Library for randomized controlled trials (RCTs) reporting on physical performance parameters after spinach extract supplementation. Study variables extracted included author, publication date, study year, number of patients, male/female ratio, mean age, mean follow-up, dose regimen, pre- and post-intervention patient-reported outcomes, and complications. Four RCTs (three red spinach, one green spinach), with 94 patients (45.7% male, 54.3% female), mean age of 29.4 years (20.5 to 58.9) and mean follow-up time of 43.8 days (7 to 84). Dosages ranged from 1 to 2 g. Spinach extract demonstrated significant improvements in most physical performance parameters, mixed results on body composition, and no complications were reported. Spinach extract demonstrates promising improvements in physical performance and safety profile as an ergogenic aid. However, more research is required to determine optimal dosing regimens and their effects in different patient populations.