Phenethylamines (PEAs) are popular substances found in weight-loss and sports nutrition supplements. They are generally pharmacologically active and primarily affect the sympathetic nervous system. Many PEAs are synthetic chemicals and are on the prohibited list of the World Anti-Doping Agency. In this study, nuclear magnetic resonance (NMR) spectroscopy was applied to detect and identify the presence of PEAs in sports dietary supplements without the need for chromatographic separation or pre-knowledge on formulation. Eight PEAs, viz. phenethylamine, synephrine, oxilofrine, hordenine, β-methylphenethylamine, N-methyltyramine, octopamine and deterenol, were identified from 32 dietary supplements sold in the US market. Furthermore, a quantitative NMR method was developed and validated for simultaneous determination of the concentrations of the PEAs. The study demonstrated that NMR could be a potential tool to monitor and detect PEAs or other ingredients in dietary supplements.