High-frequency monitoring of soluble reactive phosphorus (SRP) is essential for managing eutrophication in developing regions, but laboratory-based UV-Vis spectrophotometry (SPEC) is often limited by cost and logistics. This study evaluates the portable Reflectoquant™ (REFL) reflectometric method against SPEC using synthetic standards and 300 filtered river samples from Uruguayan watersheds (0.010–2.400 mgPO₄-P L-1). Statistical analyses included Spearman correlation, regression models, and Bland-Altman plots. Strong monotonic relationships were found (ρ = 0.979 for standards; ρ = 0.890 for rivers). Simple linear regression showed the best fit (adjusted R² ≥ 0.972; root mean square error (RMSE) ≤ 0.018 mgPO₄-P L-1). Bland-Altman analysis indicated minor overall bias (+0.009 mg L-1), with overestimation at trace levels (<0.025 mg L⁻¹; bias –0.022 mg L⁻¹) and underestimation at higher concentrations (≥0.100 mg L-1; bias +0.015 mg L-1). While sensitivity is limited at trace levels, REFL’s portability and low cost make it an effective field-screening tool in Uruguay, where SRP accounts for 17–100% of total phosphorus and often exceeds the 0.025 mg L-1 eutrophication threshold. These results support decentralized monitoring networks and advance United Nations Sustainable Development Goals 6 and 14 in resource-limited settings.