Introduction: Stress is a complex life occurrence essential for survival and goal achievement but can be damaging in excess. Because of the high prevalence of stress in North America, a safe supplement that effectively reduces stress is in demand. The objective of this study was to investigate the efficacy and safety of AlphaWave®l-Theanine on whole-scalp and frontal alpha power, midline theta power, and salivary cortisol in healthy, moderately stressed adults. Methods: This was a randomized, triple-blind, placebo-controlled, crossover study that consisted of two study periods with a 7-day washout. A single dose of AlphaWave®l-Theanine (200 mg) or placebo was administered. To induce stress, a mental arithmetic test (MAT) was administered before and after the dose. Electroencephalogram, salivary cortisol, blood pressure, heart rate, self-reported stress, adverse events, clinical chemistry, and hematology were assessed to evaluate efficacy and safety. Results: Increases in heart rate, blood pressure, and self-reported stress and state anxiety indicated that participants experienced stress during the MAT. AlphaWave®l-Theanine led to a greater increase in frontal region and whole-scalp alpha power 3 h post-dose compared to placebo (p ≤ 0.050). Within groups, there were increases in alpha power, at 3 h with AlphaWave®l-Theanine, over the whole recording and during the eyes-open portions (p ≤ 0.048) of the alpha task. The changes in alpha wave activity are supported by greater decreases in salivary cortisol 1 h post-dose (p < 0.001) with AlphaWave®l-Theanine compared to placebo. Conclusion: This study was conducted during the SARS-CoV-2 pandemic, which has had a rapid and significant effect on both physical and mental health around the world. A single dose of AlphaWave®l-Theanine significantly increased frontal region alpha power compared to placebo in response to an acute stress challenge. These changes are indicative of relaxation in the brain and suggest a calming response. AlphaWave®l-Theanine was found to be safe and well tolerated by participants. Trial Registration: ClinicalTrials.gov identifier NCT04706494. Plain Language Summary: Stress is a complex part of life that is essential for survival and achieving goals. Too much stress, however, can be damaging. There is a high prevalence of stress in North America, creating a demand for a safe and effective supplement to reduce it. This study investigated the effectiveness and safety of AlphaWave®l-Theanine on brainwaves and salivary cortisol in healthy, moderately stressed adults facing an acute stressor. This was a randomized, triple-blind, placebo-controlled, crossover study that consisted of two study periods with a 7-day washout. A single dose of 200 mg of AlphaWave®l-Theanine or placebo was administered before and after a mental arithmetic test to elicit acute stress. Electroencephalogram, salivary cortisol, blood pressure, heart rate, self-reported stress, and safety were assessed to evaluate efficacy and safety. This study was conducted during the coronavirus pandemic, which has had a rapid and significant effect on both physical and mental health around the world. A single dose of AlphaWave®l-Theanine had significant positive effects on brainwaves, salivary cortisol, and self-reported state anxiety compared to the placebo in response to an acute stress challenge. These changes are indicative of relaxation in the brain and suggest a calming response in a moderately stressed but otherwise healthy population. AlphaWave®l-Theanine was found to be safe and well tolerated by participants. [ABSTRACT FROM AUTHOR]