This randomized controlled trial investigated the effect of a brief unsupported behavioral activation Internet intervention (BAII) designed to improve mood. A total of 671 participants were recruited through Amazon’s Mechanical Turk (AMT) (M age = 35.71 years; SD = 12.34, 32.3% male) and 187 (27.9%) individuals completed the 1-week follow-up survey. Participants were randomly assigned to a behavioral activation “Activities” condition or a waitlist control (WLC) and assessed for depression (PHQ-9), anxiety (GAD-7), self-reported mood, and confidence and motivation to make behavioral changes. Repeated measure ANOVAs showed that individuals in the Activities condition significantly improved their motivation and confidence from baseline to immediate-post intervention. Mixed-effect modeling was used to determine the within-subject effect (time) and between-subject effect (WL vs. Activities condition). Significant main effects for time (but not condition) were found for PHQ-9 depression scores (F (1, 393) = 13.24, p < 0.001), GAD-7 anxiety scores (F 1, 393) = 8.36, p < 0.01), and for subjective ratings of confidence (F (1, 387) = 4.94, p < 0.05), such that depression and anxiety scores were lower and confidence was higher at the second time-point, regardless of condition. Furthermore, a significant time by condition interaction was found for self-reported mood (F (1, 387) = 4.24, p < 0.05), where individuals who received the intervention had a more positive rating of subjective mood at the 1-week follow-up. Individuals in the WL condition did not report significant improvements in subjective mood at follow-up (p = 0.056). After a brief online behavioral activation intervention, participants who endorsed mild to moderate depression symptoms at baseline experienced an immediate improvement in confidence and motivation. They also exhibited significant improvements in self-reported mood at 1 week. However, there was no change observed in depression, anxiety, motivation, or confidence at 1-week follow-up. Overall, this study provides partial support for the utilization of a brief online activities-focused intervention and highlights the need to develop more powerful versions of such interventions.